Repo: Add a CI check that 'breaking change' PR descriptions start with the right body text

### Suggestion

Per #5874, breaking change PRs need to start with a description like:

```
BREAKING CHANGE:
One line description of the change
```

Example PR: #5036

Let's add in a CI check that, when a PR has the `breaking changes` label, its body should match that regexp.

I wonder if there's some app we can install for it, with a configurable label -> regex mapping?