ci: generate knip reports

[04kash]

Hey, I just made a Pull Request!

Added a new job, generate-knip-reports, to the ci.yml workflow which generates a knip report for each plugin in all workspaces that have been changed and creates a new PR with these knip reports.

Fixes: https://github.com/backstage/community-plugins/issues/626

:heavy_check_mark: Checklist

• [ ] A changeset describing the change and affected packages. (more info)
• [ ] Added or updated documentation
• [ ] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [X] All your commits have a Signed-off-by line in the message. (more info)