ci: replace our pullapprove setup with GitHub CODEOWNERS (#27690)

Summary of changes:
- created .github/CODEOWNERS with docs and config similar to the one in .pullapprove.yml
- updated docs
- updated .github/angular-robot.yml to not expect pullapprove status
- removed .pullapprove.yml

The primary motivations behind this change are:
- CODEOWNERS didn't exist when we introduced pullapprove
- CODEOWNERS is a functionality tightly integrated with github which results in better DX
- pullapprove v2 has been very unstable recently causing productivity loss
- pullapprove v2 has been deprecated in favor of v3, which requires and migration

PR Close #27690

.github/angular-robot.yml

@@ -81,7 +81,7 @@ merge:
     # require that the PR has reviews from all requested reviewers
     #
     # This enables us to request reviews from both eng and tech writers, or multiple eng folks, and prevents accidental merges.
-    # Rather than merging PRs with pending reviews, if all PullApprove requirements are satisfied and additional reviews are not needed pending reviewers should be removed via GitHub UI (this also leaves an audit trail behind these decisions).
+    # Rather than merging PRs with pending reviews, if all approvals are obtained and additional reviews are not needed, any pending reviewers should be removed via GitHub UI (this also leaves an audit trail behind these decisions).
     requireReviews: true,
 
     # whether the PR shouldn't have a conflict with the base branch
@@ -102,7 +102,6 @@ merge:
     # list of PR statuses that need to be successful
     requiredStatuses:
       - "continuous-integration/travis-ci/pr"
-      - "code-review/pullapprove"
       - "ci/circleci: build"
       - "ci/circleci: lint"
 

docs/COMMITTER.md

@@ -8,7 +8,7 @@ Someone with committer access will do the rest.
 
 # Change approvals
 
-Change approvals in our monorepo are managed via [pullapprove.com](https://about.pullapprove.com/) and are configured via the `.pullapprove.yaml` file.
+Change approvals in our monorepo are managed via [GitHub CODEOWNERS](https://help.github.com/articles/about-codeowners/) and are configured via the `.github/CODEOWNERS` file.
 
 
 # Merging

docs/TRIAGE_AND_LABELS.md

@@ -145,7 +145,7 @@ If a PR is missing the "PR target: *" label, or if the label is set to "TBD" whe
 
 Before a PR can be merged it must be approved by the appropriate reviewer(s).
 
-To ensure that there right people review each change, we configured [PullApprove bot](https://about.pullapprove.com/) via (`.pullapprove.yaml`) to provide aggregate approval state via the GitHub PR Status API.
+To ensure that the right people review each change, we configured [GitHub CODEOWNERS](https://help.github.com/articles/about-codeowners/) (via `.github/CODEOWNERS`) and require that each PR has at least one approval from the appropriate code owner.
 
 Note that approved state does not mean a PR is ready to be merged.
 For example, a reviewer might approve the PR but request a minor tweak that doesn't need further review, e.g., a rebase or small uncontroversial change.