Fix RAPIDS_BRANCH file

[KyleFromNVIDIA]

Description

Set it to branch-25.12.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[jameslamb]

Approving, but just to note... it looks like this has fallen through and not matched any CODEOWNERS rules.

It's be useful to have a catch-all CODEOWNERS rule here, something like:

* @rapidsai/rapids-cmake-codeowners

At the top of the file (so it'd be superseded by more specific matches that follow).