Add post dependabot action to update notice file

.github/workflows/post-dependabot.yml

@@ -0,0 +1,57 @@
+# Follow-on actions relating to dependabot PRs. In elastic/fleet-server, any changes to
+# dependencies contained in go.mod requires the change to be reflected in the
+# NOTICE.txt file. When dependabot creates a branch for a go_modules change this
+# will update the NOTICE.txt file for that change.
+name: post-dependabot
+
+on:
+  push:
+    branches:
+      - 'dependabot/go_modules/**'
+
+jobs:
+  update-notice:
+    permissions:
+      # Allow job to write to the branch.
+      contents: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - uses: actions/setup-go@v4
+        with:
+          go-version-file: .go-version
+
+      - name: run go mod tidy in testing/
+        run: cd testing; go mod tidy
+
+      - name: check for modified testing/go.mod or testing/go.sum
+        id: testing-mod-check
+        run: echo "modified=$(if git diff-index --quite HEAD -- testing/go.mod testing/go.sum; then echo "false"; else echo "true"; fi)" >> $GITHUB_OUTPUT
+
+      - name: commit testing/go.mod and testing/go.sum files
+        if: steps.testing-mod-check.outputs.modified == 'true'
+        run: |
+          git config --global user.name 'dependabot[bot]'
+          git config --global user.email 'dependabot[bot]@users.noreply.github.com'
+          git add testing/go.mod testing/go.sum
+          git remote set-url origin https://x-access-token:${{ secrets.GITHUB_TOKEN }}@github.com/${{ github.repository }}
+          git commit -m "Update testing/go.mod and testing/go.sum files"
+          git push
+
+      - name: update NOTICE.txt
+        run: make notice
+
+      - name: check for modified NOTICE.txt
+        id: notice-check
+        run: echo "modified=$(if git diff-index --quiet HEAD -- NOTICE.txt; then echo "false"; else echo "true"; fi)" >> $GITHUB_OUTPUT
+
+      - name: commit NOTICE.txt
+        if: steps.notice-check.outputs.modified == 'true'
+        run: |
+          git config --global user.name 'dependabot[bot]'
+          git config --global user.email 'dependabot[bot]@users.noreply.github.com'
+          git add NOTICE.txt
+          git remote set-url origin https://x-access-token:${{ secrets.GITHUB_TOKEN }}@github.com/${{ github.repository }}
+          git commit -m "Update NOTICE.txt"
+          git push