Make ECK 2.15.0 the current version

[rhr323]

This PR sets the ECK 2.15 branch as the current version.

This PR depends on #3100 which should be merged first. It's submitted early only to get approval, I'll rebase on Wed, November 06 once #3100 is merged.

Do not merge before ECK 2.15 release date (November 7, 2024).

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[rhr323]

Hey @szabosteve, @leemthompo, @shainaraskas , @elastic/docs-repo-owners ,
We’ve successfully resolved the issue in this PR, and the build has now passed. I’d appreciate it if you could take a moment to review.
Thanks!