ci: set release-please manifest to last released version

The manifest had 4.2.0-SNAPSHOT which doesn't match any git tag,
causing release-please to compare against a much older baseline
and incorrectly propose a major version bump.

Author: osiegmar

.release-please-manifest.json

@@ -1,3 +1,3 @@
 {
-  ".": "4.2.0-SNAPSHOT"
+  ".": "4.1.0"
 }