ci: set initial version to v0.51.1

[knqyf263]

Description

The release PR shows many commits already included in previous versions. I guess it's because the latest version defined in .release-please-manifest.json is v0.51.4, which is not in the main branch, and Release Please doesn't find it. My somewhat incomplete understanding is that Release Please goes back through the commits until it finds the v0.51.4 commit and, as a result, the release PR includes commits that have already been released.

If my understanding above is correct, the problem should be solved by specifying the tags present in the main commit, so this PR changes the latest version to v0.51.1.

Related PRs

• ci: introduce Release Please for automated release management #6795

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

It looks good now.
#6809