refactor: remove platform.getPrFiles (#6702)

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

lib/platform/azure/index.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket/index.ts

lib/platform/common.ts

lib/platform/gitea/index.ts

lib/platform/github/index.ts

lib/platform/gitlab/index.ts

lib/util/git/__snapshots__/index.spec.ts.snap

lib/util/git/index.spec.ts

lib/util/git/index.ts

lib/workers/pr/code-owners.spec.ts

lib/workers/pr/code-owners.ts