Create semver labels if they don't exist

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Currently, the "semver label" is only added if the semver labels are registered beforehand

https://github.com/dependabot/dependabot-core/blob/72551e7acd730b8fdcdd1f995a759ebb1fd36b23/common/lib/dependabot/pull_request_creator/labeler.rb#L263C11-L271

It would be nice to update this logic to create them if they don't exist similarly to how we handle the language labels creation

https://github.com/dependabot/dependabot-core/blob/main/common/lib/dependabot/pull_request_creator/labeler.rb#L168-L174

## Note to maintainers

If accepted, I can work on this feature

Start in common/lib/dependabot/pull_request_creator/labeler.rb, comparing the semver label logic around lines 263-271 with the language label creation around lines 168-174. Confirm how absent labels are handled and update the semver path so the labels are created when needed; done means semver labels no longer require prior registration.

Written by the indexing model from the issue text.