refactor: drop pr.isStale (#6826)

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket/index.ts

lib/platform/common.ts

lib/platform/gitea/__snapshots__/index.spec.ts.snap

lib/platform/gitea/index.ts

lib/platform/github/__snapshots__/index.spec.ts.snap

lib/platform/github/index.spec.ts

lib/platform/github/index.ts

lib/platform/gitlab/__snapshots__/index.spec.ts.snap

lib/platform/gitlab/index.ts

lib/workers/repository/onboarding/branch/rebase.spec.ts

lib/workers/repository/onboarding/branch/rebase.ts