chore: general repo maintenance and docs cleanup (#3030)

Refresh README/docs for the current stack, add a security policy,
note the external-PR pause, and remove dead config and workflows.

.github/workflows/semantic-pull-requests.yml

@@ -16,24 +16,6 @@ jobs:
     name: Validate PR title
     runs-on: ubuntu-latest
     steps:
-      - name: Check PR creator's previous activity
-        id: check_activity
-        run: |
-          CREATOR=$(curl -s "https://api.github.com/repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}" | jq -r '.user.login')
-          ACTIVITY=$(curl -s "https://api.github.com/search/commits?q=author:${CREATOR}+repo:${{ github.repository }}" | jq -r '.total_count')
-          if [ "$ACTIVITY" -eq 0 ]; then
-            echo "::set-output name=is_new::true"
-          else
-            echo "::set-output name=is_new::false"
-          fi
-
-      - name: Count PRs created by user
-        id: count_prs
-        run: |
-          CREATOR=$(curl -s "https://api.github.com/repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}" | jq -r '.user.login')
-          PR_COUNT=$(curl -s "https://api.github.com/search/issues?q=type:pr+is:open+author:${CREATOR}+repo:${{ github.repository }}" | jq -r '.total_count')
-          echo "::set-output name=pr_count::$PR_COUNT"
-
       - uses: amannn/action-semantic-pull-request@v5
         id: lint_pr_title
         env:
@@ -44,8 +26,6 @@ jobs:
         with:
           header: pr-title-lint-error
           message: |
-            Hey There! and thank you for opening this pull request! 📝👋🏼
-
             We require pull request titles to follow the [Conventional Commits Spec](https://www.conventionalcommits.org/en/v1.0.0/) and it looks like your proposed title needs to be adjusted.
 
             Details:
@@ -53,10 +33,3 @@ jobs:
             ```
             ${{ steps.lint_pr_title.outputs.error_message }}
             ```
-
-      - if: ${{ steps.lint_pr_title.outputs.error_message == null && steps.check_activity.outputs.is_new == 'false' && steps.count_prs.outputs.pr_count < 2}}
-        uses: marocchino/sticky-pull-request-comment@v2
-        with:
-          header: pr-title-lint-error
-          message: |
-            Thank you for following the naming conventions for pull request titles! 💚🚀