Change commit message topic to 'container image digests' (dotnet#127614)

"dependencies" is confusing or misleading. That's the default renovate
terminology. "digests" is much clearer to anyone that is in a position
to merge the generated PRs.

PR examples:
https://github.com/dotnet/runtime/pulls?q=is%3Apr+author%3Adotnet-renovate-bot+

Author: richlander

eng/renovate.json

@@ -37,7 +37,7 @@
         "eng/pipelines/libraries/helix-queues-setup.yml"
       ],
       "pinDigests": true,
-      "commitMessageTopic": "container image dependencies"
+      "commitMessageTopic": "container image digests"
     }
   ]
 }