ci: bump lib9c version automatically

[moreal]

Implementation

Note

This is just a thoughtful implementation direction. If you have a better direction or preferred implementation, you are of course free to do things differently.

• When GitHub Actions push.on: tag event, it opens a new pull request which bumps Version property in Lib9c/Lib9c.csproj file.
• It may be able to validate current validate that the Version property's value already exists in tags, in pull request checks.