chore(nns): remove eight year gang bonus base migration logic

[jasonz-dfinity]

Why

The strict and the relaxed eight year gang bonus base migrations have already
run on mainnet (proposals 141441 and 141565). Migrations don't re-run, so the
migration code and supporting constants/tests are dead weight.

What

• Remove Governance::maybe_set_eight_year_gang_bonus_base and
  maybe_set_relaxed_eight_year_gang_bonus_base_e8s_or_panic, plus the call
  site in Governance::new_restored.
• Remove the corresponding set_eight_year_gang_bonus_base_e8s_for_all_neurons_or_panic
  and set_relaxed_eight_year_gang_bonus_base_e8s_or_panic helpers from
  NeuronStore and StableNeuronStore.
• Drop RELAXED_EIGHT_YEAR_GANG_MIN_DISSOLVE_DELAY_SECONDS and
  RELAXED_EIGHT_YEAR_GANG_MAX_AGING_SINCE_TIMESTAMP_SECONDS.
• Remove the migration-only tests
  (test_maybe_set_eight_year_gang_bonus_base,
  test_maybe_set_relaxed_eight_year_gang_bonus_base,
  test_set_eight_year_gang_bonus_base_e8s_for_all_neurons,
  test_set_relaxed_eight_year_gang_bonus_base_e8s).

eight_year_gang_bonus_base_e8s is still maintained on the regular path: new
neurons default it to 0, start_dissolving zeroes it, and split/merge keeps
it correctly partitioned across parent/child neurons.

Rollback safety

The two *_migration_done flags are kept on HeapGovernanceData and in the
proto as persisted-true rollback guards. If we ever rolled back to a release
that still contains the migration code, the strict migration would otherwise
re-run against post-Mission-70 (clamped ≤ 2-year) dissolve delays, hit the
else branch for every neuron, and zero out every neuron's
eight_year_gang_bonus_base_e8s. Keeping the flags persisted as true makes
the previous-release migration short-circuit on rollback.

[Copilot]

Pull request overview

This PR removes now-dead “eight year gang bonus base” one-time migration logic from NNS Governance after confirming the strict and relaxed migrations have already executed on mainnet, and updates the persisted protobuf schema accordingly.

Changes:

• Removed the strict + relaxed migration implementations and their call site during Governance::new_restored.
• Deleted the migration-only helper APIs from the neuron stores and removed migration-only tests.
• Dropped the persisted *_migration_done flags and reserved protobuf field numbers 31 and 33 to preserve schema compatibility.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rs/nns/governance/unreleased_changelog.md	Documents removal of the one-time migrations and protobuf field reservation.
rs/nns/governance/src/storage/neurons/neurons_tests.rs	Removes migration-only tests and now-unused import.
rs/nns/governance/src/storage/neurons.rs	Deletes stable neuron store migration helpers and related imports.
rs/nns/governance/src/neuron_store/neuron_store_tests.rs	Removes relaxed-migration-only test and now-unused imports.
rs/nns/governance/src/neuron_store.rs	Deletes migration helper methods and related imports/constants usage.
rs/nns/governance/src/heap_governance_data.rs	Removes migration flags from heap governance state split/reassemble paths and tests.
rs/nns/governance/src/governance/tests/mod.rs	Removes migration-specific tests and now-unused imports.
rs/nns/governance/src/governance.rs	Removes migration constants and the post-upgrade migration entry points.
rs/nns/governance/src/gen/ic_nns_governance.pb.v1.rs	Updates generated protobuf Rust bindings to drop the deprecated fields.
rs/nns/governance/proto/ic_nns_governance/pb/v1/governance.proto	Removes deprecated fields and reserves tags/names to prevent reuse.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

How to Satisfy This Automatic Review

1. Go to the bottom of the pull request page.

2. Look for where it says this bot is requesting changes.

3. Click the three dots to the right.

4. Select "Dismiss review".

5. In the text entry box, respond to each of the numbered items in the previous
   section, declare one of the following:

• Done.

• $REASON_WHY_NO_NEED. E.g. for unreleased_changelog.md, "No
  canister behavior changes.", or for item 2, "Existing APIs
  behave as before.".

Brief Guide to "Externally Visible" Changes

"Externally visible behavior change" is very often due to some NEW canister API.

Changes to EXISTING APIs are more likely to be "breaking".

If these changes are breaking, make sure that clients know how to migrate, how to
maintain their continuity of operations.

If your changes are behind a feature flag, then, do NOT add entrie(s) to
unreleased_changelog.md in this PR! But rather, add entrie(s) later, in the PR
that enables these changes in production.

Reference(s)

For a more comprehensive checklist, see here.

GOVERNANCE_CHECKLIST_REMINDER_DEDUP

N/A for cleanup