chore: add Datadog Code Coverage upload, remove CodeCov

Replace the CodeCov upload step in coverage.yml with a Datadog Code
Coverage upload using @datadog/datadog-ci. Rename the workflow from
"codecov" to "coverage".

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nikita-tkachenko-datadog

.github/workflows/coverage.yml

@@ -1,4 +1,4 @@
-name: codecov
+name: coverage
 on: [push, pull_request]
 
 permissions:
@@ -22,8 +22,7 @@ jobs:
       - name: Run coverage with new pickfirst
         run: GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true go test -coverprofile=coverage_new_pickfirst.out -coverpkg=./... ./...
 
-      - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v4
-        with:
-          token: ${{ secrets.CODECOV_TOKEN }}
-          fail_ci_if_error: true
+      - name: Upload coverage to Datadog
+        env:
+          DATADOG_API_KEY: ${{ secrets.DATADOG_API_KEY }}
+        run: npx --yes @datadog/datadog-ci coverage upload --service grpc-go coverage.out