github(codeowners): robots-ci can review .github/workflows only (#5112)

Author: v1v

.github/CODEOWNERS

@@ -4,4 +4,4 @@
 # Allow to auto-merge PRs with Mergify
 dev-tools/integration/.env
 
-/.github @elastic/observablt-ci @elastic/observablt-ci-contractors
+/.github/workflows @elastic/observablt-ci