chore(release): remove the 0.28.6 release pin

Remove the root-level release-as so release-please computes versions
from commit types again. Merge only after 0.28.6 is tagged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: declan-scale

release-please-config.json

@@ -33,7 +33,6 @@
     }
   ],
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
-  "release-as": "0.28.6",
   "include-v-in-tag": true,
   "include-component-in-tag": true,
   "bump-minor-pre-major": true,