Add "Update with rebase" button

We were seeing 2 or 3 cases when merging two pull requests results in a non-compilable code in main branch. This is because PRs are usually not rebased.

I have now enabled a button to update a PR to the tip of the branch. Previously this button added a merge commit, which I don't like. Now there's an option to do a rebase with force push. Unfortunately this is a second option so you have to explicitly select it.

⚠️ @opencontainers/runc-maintainers, please use the "Update with rebase" option, NOT the default "Update with merge commit".

Here's a short blog post by github about the new feature: https://github.blog/changelog/2022-02-03-more-ways-to-keep-your-pull-request-branch-up-to-date/

I suggest all admins to use this before merging a PR. I have tried it myself on https://github.com/opencontainers/runc/pull/3404 and it works as expected (no merge commit is created, no LGTMs are lost).

Pros:

• 🟢 ability to test a PR against tip of the branch
• 🟢 no LGTMs are lost
• 🟢 no merge commit is created

Cons:

• 🔴 if the author wants to work on the branch, they have to do a fetch and a hard reset (or a local git pull --rebase)

Because of 🔴, I think the best way to use this feature is when the PR is ready to be merged, i.e. no more changes are expected.

No repository files or tests are named. Start by reviewing the issue discussion and GitHub's documented "Update with rebase" behavior, then determine whether maintainers want a repository change or only a workflow decision. The issue does not define a code-level completion criterion.