chore: Run release-please only on open-telemetry

Currently, release-please attempts to run on forks of
opentelemetry-ruby-contrib. The workflow fails with the message
"GitHub Actions is not permitted to create or approve pull requests."
release-please is not needed on forks.

.github/workflows/release-please.yaml

@@ -13,6 +13,7 @@ permissions:
 
 jobs:
   prepare:
+    if: ${{ github.repository == 'open-telemetry/opentelemetry-ruby-contrib' }}
     name: Process Release
     runs-on: ubuntu-latest
     steps: