[llvm] github-automation.py: Set maintainer_can_modify=True for backport PRs (PR #84819)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM. This also allows people to fix issues with a backport without opening a new PR...

https://github.com/llvm/llvm-project/pull/84819