operatorhub: updated source branch for channel

[AndrewChubatiuk]

in release 0.68.5 channel wasn't set correctly since it used tag as a source, which is not correct

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Summary by cubic

Fixes OperatorHub channel selection by deriving it from the release target branch, not the tag. Bundles now publish to the correct channel.

• Bug Fixes
  • In release.yaml, write bundle/CHANNEL as "stable" when target_commitish starts with release-, else "beta".
  • In operatorhub.yaml, read bundle/CHANNEL (default "beta"), set both CSV and bundle metadata channel annotations to CHANNEL, generate catalog templates with only "stable" when CHANNEL=stable, and remove the old head-branch logic and the file.

^{Written for commit a3e9e85. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}