Rewrite the releases developer guide

[ideaship]

The releases developer guide still described the release process of the
5.x/6.x era and had drifted far from current practice. This rewrite
brings it in line with how the 9.x and 10.0.0 releases were actually
made.

What changed

• Removed obsolete content:
  • Pre-release directory mechanics (copying latest/, stripping
    renovate lines, fixing symlinks, prepare-release.py) — replaced
    by src/create-version.py and -rc.N release candidates.
  • Release-name tags on the container image repositories — replaced by
    date-based component tags created with scripts/create-tags.sh.
  • All SBOM steps — the osism/sbom repository is archived.
  • The Reno release notes
    section — Reno is no longer used in the component repositories.
• Linked to the
  README of the osism/release repository
  for the in-repo mechanics (Renovate updates, tagging, freezing a
  release, changelog generation) instead of duplicating them.
• Kept what that README does not cover: the release policy (major/minor
  semantics and upgrade guarantees) and the cross-repo release checklist
  (testbed release candidate jobs, release notes in the documentation,
  version examples in the documentation, the cfg-cookiecutter
manager_version default, switching the testbed stable jobs as the
  final step).

Needs confirmation

Marked inline with warning admonitions, because the new content was
reverse engineered from git history rather than from a documented
process:

• The release cadence (the old "every 6 months / every 2 weeks" did not
  match observed practice).
• The completeness and order of the release checklist.
• Whether GitHub releases on osism/release are permanently retired
  (last one was 8.0.2), and what replaces them as the public
  announcement.
• That no component repository still relies on Reno.

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	4		0	0	0.04s
✅ JSON	jsonlint	4		0	0	0.14s
✅ JSON	prettier	4		0	0	0.45s
✅ JSON	v8r	4		0	0	8.58s
⚠️ MARKDOWN	markdownlint	160		4	0	2.78s
⚠️ MARKDOWN	markdown-table-formatter	160		1	0	0.47s
✅ REPOSITORY	checkov	yes		no	no	19.08s
✅ REPOSITORY	git_diff	yes		no	no	0.07s
✅ REPOSITORY	secretlint	yes		no	no	1.77s
✅ REPOSITORY	trufflehog	yes		no	no	3.9s
✅ SPELL	codespell	169		0	0	0.6s
⚠️ SPELL	lychee	169		2	0	11.7s
✅ YAML	prettier	5		0	0	0.44s
✅ YAML	v8r	5		0	0	6.23s
✅ YAML	yamllint	5		0	0	0.57s

Detailed Issues

⚠️ SPELL / lychee - 2 errors

📝 Summary
---------------------
🔍 Total..........926
🔗 Unique.........744
✅ Successful.....863
⏳ Timeouts.........0
🔀 Redirected......47
👻 Excluded........61
❓ Unknown..........0
🚫 Errors...........2
⛔ Unsupported......2

Errors in docs/release-notes/osism-7.md
[ERROR] https://scs.community/de/security/2024/07/23/cve-2024-40767/ (at 134:5) | Network error: Connection reset by peer (os error 104) | Followed 1 redirect. Redirects: https://scs.community/de/security/2024/07/23/cve-2024-40767/ --[301]--> https://sovereigncloudstack.org/community_blog/scs-security-advisory-on-incomplete-qcow2-and-vmdk-image-handling-protections-cve-2024-40767/
[ERROR] https://scs.community/de/security/2024/09/07/cve-2024-44082/ (at 40:5) | Network error: Connection reset by peer (os error 104) | Followed 1 redirect. Redirects: https://scs.community/de/security/2024/09/07/cve-2024-44082/ --[301]--> https://sovereigncloudstack.org/community_blog/sovereign-cloud-stack-security-advisory-image-processing-in-ironic-cve-2024-44082/

Hint: Followed 47 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

⚠️ MARKDOWN / markdown-table-formatter - 1 error

1 files contain markdown tables to format:
- docs/guides/developer-guide/retiring-openstack-releases.md

⚠️ MARKDOWN / markdownlint - 4 errors

docs/guides/developer-guide/retiring-openstack-releases.md:181:450 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]
docs/guides/developer-guide/retiring-openstack-releases.md:181:535 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]
docs/guides/developer-guide/retiring-openstack-releases.md:183:417 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]
docs/guides/developer-guide/retiring-openstack-releases.md:183:502 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]

Notices

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