CI: Allow backporting to arbitrary branch

[leofang]

Right now, we hard-code the backport branch name here:
https://github.com/NVIDIA/cuda-python/blob/main/.github/BACKPORT_BRANCH
We should relax it and allow:

• triggering the backport workflow manually
• providing a branch name to overwrite the default above

[rparolin]

Closing as done. PR #1469 added workflow_dispatch with backport-branch and pull-request inputs (backport.yml:12-21) plus the backport-to-branch job (:52-76), and it was used in production the same day — #1471 through #1476 backported to release/cuda-core-v0.5.1, a non-default branch.

The premise is also stale: .github/BACKPORT_BRANCH, which this issue links, was deleted by #1311 in favour of ci/versions.yml.