Remove long running per-PR CI (ideally keeping only checks <10 minutes)

[pearigee]

We shouldn't be running long-running integration tests for every PR. This creates unnecessary toil for contributors and burns are shared quota at an accelerated rate.

[pearigee]

Progress has been made on this issue via:

• chore: disable sample tests for packages/* #7996
  • We have disabled sample tests for generated packages. These tests call service RPCs with an empty request (essentially operating as a service smoke test). These are flaky and seem to add very little value.
• chore: disable system tests in packages to unblock PRs pending fix #7974
  • We have disabled the flakey system tests for our generated packages. These system tests take forever to run,but essentially just create the client and then delete it. No behavior is tested.

Per #7921 we are planning to simply remove these tests going forward.

[pearigee]

The core-system-presubmit-node CI is now blocking Firestore. I think it's time to rip the bandaid off and disable these CIs for most the respository.

Firestore doesn't even touch our core libraries.

I am going to request AOD access and make the change.

[pearigee]

I have modified the branch rules to make it so that core system and sample tests are no longer required (i.e. blocking) for PR submission.

Note, upon further investigation, all of the these tests are being skipped anyway during CI execution, see https://github.com/googleapis/google-cloud-node/blob/main/ci/run_conditional_tests.sh#L111 (they were disabled during the monorepo migration).

Despite that, the actually execution of these tests can still fail during there initialization/build step.

---

Follow up investigation is needed to determine if the core system/sample tests are worth repairing.

[pearigee]

I have disabled the "packages-system-presubmit-node" GCB trigger here: https://pantheon.corp.google.com/cloud-build/triggers;region=global/edit/fc5c25d5-4b77-468d-9316-e9dc9bfe1079?project=long-door-651

This should prevent it from triggering on PR CI.

[pearigee]

Similarly, the system and sample tests have been disabled for the core directory: https://github.com/googleapis/google-cloud-node/blob/main/ci/run_conditional_tests.sh#L111

This appears to have been done during the monorepo migration. We need to reassess the value added and possibly repair them before they can be useful.

However, despite them being skipped, this still depends on the cloud build executing to run this bash script. This is just a waste of resources and pollutes per-PR CI.

I have disabled them as well until such time as they can be repaired:

• https://github.com/googleapis/google-cloud-node/blob/main/ci/run_conditional_tests.sh#L111
• https://pantheon.corp.google.com/cloud-build/triggers;region=global/edit/cd0eedc4-7379-4d88-9a34-ce1f5190d709?project=long-door-651
• https://pantheon.corp.google.com/cloud-build/triggers;region=global/edit/fde91b53-a2f5-4b0b-b1ae-b8c193c8365e?project=long-door-651

[pearigee]

The Kokoro tests (while still running on every PR) are disabled if the current PR doesn't contain changes to the test (or disabled entirely if flaky).

Once we migrate to from Kokoro to GCB, we can also prevent the CI from running on every PR, just to determine if it should run or not.