Auto rebasing support for collabora-next tree

[laura-nao]

Collabora maintains a linux-next based tree on Gitlab for testing patches in KernelCI that haven’t been merged upstream: https://gitlab.collabora.com/kernel/collabora-next

This tree is built on linux-next and needs to be periodically rebased, ideally every time a new linux-next tag is released. Currently, this rebase is done manually (e.g., git fetch <next-remote> master; git rebase --onto <next-remote>/master HEAD~3).

This issue proposes implementing automatic rebasing for the for-kernelci branch of the collabora-next tree, which would enable KernelCI tests to be triggered without manual intervention. Automating this process could also benefit other kernel trees.