chore: revert Librarian CLI release to pre-major release (#2898)

We prematurely released 1.0.0 version of Librarian CLI, when it is not
yet stable. We have retracted this release and will continue on with pre
1.0 releases, starting with 0.6.0.

Author: ldetmer

.librarian/state.yaml

@@ -1,7 +1,7 @@
 image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/librarian-release-container:latest
 libraries:
   - id: librarian
-    version: 1.0.0
+    version: 0.6.0
     last_generated_commit: 97a83d76a09a7f6dcab43675c87bdfeb5bcf1cb5
     apis: []
     source_roots: