chore(main): release 0.306.0 (#8654)

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

Author: release-please[bot]

VERSION

@@ -1 +1 @@
-1.106.0
+2.0.0-RC1

src/SpannerClient.php

@@ -113,7 +113,7 @@ class SpannerClient
     use ApiHelperTrait;
     use RequestTrait;
 
-    const VERSION = '1.106.0';
+    const VERSION = '2.0.0-RC1';
 
     const FULL_CONTROL_SCOPE = 'https://www.googleapis.com/auth/spanner.data';
     const ADMIN_SCOPE = 'https://www.googleapis.com/auth/spanner.admin';