Fix CI workflow for integration tests

- Remove unused services block that caused CI failure
  (Gitea runner doesn't support --name/-p in options field)
- Update build tag to modern //go:build syntax (Go 1.17+)

The workflow already manually installs and starts NATS with JetStream,
making the services block redundant.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

store/jetstream_integration_test.go

@@ -1,4 +1,4 @@
-// +build integration
+//go:build integration
 
 package store