[21244] Include v2.6.9 release notes and 2.6.x maintenance flag (backport #848)

[mergify]

Description

This PR adds the previous notes from v2.6.9 and also marks the 2.6.x branches as maintenance

@Mergifyio backport 2.14.x 2.13.x 2.10.x

Contributor Checklist

• Commit messages follow the project guidelines.
• Code snippets related to the added documentation have been provided.
• Documentation tests pass locally.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

---

This is an automatic backport of pull request #848 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 433dc63 has failed:

On branch mergify/bp/2.13.x/pr-848
Your branch is up to date with 'origin/2.13.x'.

You are currently cherry-picking commit 433dc63.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   docs/notes/previous_versions/v2.6.9.rst
	modified:   docs/spelling_wordlist.txt

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/notes/previous_versions/supported_versions.rst
	deleted by us:   docs/notes/versions.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[JesusPoderoso]

LGTM with green CI