Improved release branch prep python script

[mbianchidev]

This pull request improves the check_version function in releng/prepare_release_branch.py by adding stricter validation for release configuration file names and enhancing error handling. The changes ensure that only valid version numbers are processed, provide clearer error messages, and warn about invalid file names when found.

Enhancements to version validation and error handling:

• Added a regex check to ensure file names match the x.y.yaml format, skipping invalid files and issuing a warning with details about the skipped files.
• Introduced an error message when no valid version numbers are found, prompting users to check the config directory.
• Improved the sequential version check by providing detailed error messages, including the expected and found version numbers.
• Added a log message to confirm valid version numbers found, for debug/verification reasons.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mbianchidev
Once this PR has been reviewed and has the lgtm label, please assign justaugustus for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• releng/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment