releases: add the seed image step, move all generator defaults

[ideaship]

Two gaps in the release checklist (docs/guides/developer-guide/releases.md):

• Seed image. The seed image in osism/container-images carries one collection set per release listed in seed/files/releases.txt, and run.sh refuses a manager_version it has no set for. The checklist never mentioned that list. A new step after creating the final release lists the release and waits for the published seed image before the testbed stable jobs are switched; step 2 points at it for release candidates. The later steps are renumbered.
• Generator defaults. The last step bumps only manager_version in osism/cfg-cookiecutter. On a new OpenStack series openstack_version has to move with it, because the generated scripts/generate-secrets.sh selects environments/kolla/secrets.yml.<version> by it, and ceph_version moves when the default Ceph release changes.

Related:

• seed: Add 10.3.0 release container-images#1002
• Default to OSISM 11 with 2026.1 and Tentacle cfg-cookiecutter#885

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ ACTION	actionlint	5		0		0	0.04s
✅ JSON	jsonlint	4		0		0	0.08s
✅ JSON	prettier	4		0		0	0.33s
✅ JSON	v8r	4		0		0	10.95s
✅ MARKDOWN	markdownlint	171		0		0	2.81s
✅ MARKDOWN	markdown-table-formatter	171		0		0	0.39s
✅ REPOSITORY	betterleaks	yes		no		no	0.73s
✅ REPOSITORY	checkov	yes		no		no	19.31s
✅ REPOSITORY	git_diff	yes		no		no	0.07s
✅ REPOSITORY	secretlint	yes		no		no	1.63s
✅ REPOSITORY	trufflehog	yes		no		no	4.73s
✅ SPELL	codespell	181		0		0	0.65s
⚠️ SPELL	lychee	181		1		0	60.32s
✅ YAML	prettier	6		0		0	0.33s
✅ YAML	v8r	6		0		0	8.53s
✅ YAML	yamllint	6		0		0	0.52s

Detailed Issues

⚠️ SPELL / lychee - 1 error

📝 Summary
---------------------
🔍 Total.........1173
🔗 Unique.........884
✅ Successful....1071
⏳ Timeouts.........0
🔀 Redirected.......9
👻 Excluded.......101
❓ Unknown..........0
🚫 Errors...........1
⛔ Unsupported......1

Errors in docs/guides/deploy-guide/metalbox.md
[404] https://github.com/osism/metalbox/blob/main/zuul/vars/container-images-openstack-2024.2.yml (at 83:101) | Rejected status code: 404 Not Found

Hint: Followed 9 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

See detailed reports in MegaLinter artifacts

Your project could benefit from a custom flavor, which would allow you to run only the linters you need, and thus improve runtime performances. (Skip this info by defining FLAVOR_SUGGESTIONS: false)

• Documentation: Custom Flavors
• Command: npx mega-linter-runner@10.1.0 --custom-flavor-setup --custom-flavor-linters ACTION_ACTIONLINT,JSON_JSONLINT,JSON_V8R,JSON_PRETTIER,MARKDOWN_MARKDOWNLINT,MARKDOWN_MARKDOWN_TABLE_FORMATTER,REPOSITORY_CHECKOV,REPOSITORY_GIT_DIFF,REPOSITORY_BETTERLEAKS,REPOSITORY_SECRETLINT,REPOSITORY_TRUFFLEHOG,SPELL_LYCHEE,SPELL_CODESPELL,YAML_PRETTIER,YAML_YAMLLINT,YAML_V8R

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