Use `cancel-in-progress` instead of `styfle/cancel-workflow-action`.

Advantages:
- We don't need to grant write permissions to a third-party action (or even use it at all).
  - Not that I have any reason to think it's likely to be a problem, but strange things happen sometimes.
- One less action means one less thing for us to merge Dependabot updates for.
- The new approach can easily allow CI for merged trunk commits to continue even after other trunk commits are merged, while still stopping PR CI when new changes are pushed.
  - This is especially nice because cancellations (at least those from `styfle/cancel-workflow-action`, maybe in general) show up as _failures_. It's nice to be able to pick out _real_ failures (e.g., those fixed by cl/925609698 and cl/943404496) without sorting through cancellations.
- All else being equal, it's nice to have smaller, simpler configuration.

Assuming that this approach holds up, I'll replicate it to our other projects.

RELNOTES=n/a
PiperOrigin-RevId: 943469061