fix: use config-file/manifest-file inputs so version-file is honoured

The release-type: simple input caused the action to ignore
release-please-config.json, defaulting to version.txt instead
of VERSION. Switching to explicit config-file + manifest-file
inputs forces the action to read all settings from the config.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: locus313

.github/workflows/release.yml

@@ -15,5 +15,5 @@ jobs:
     steps:
       - uses: googleapis/release-please-action@45996ed1f6d02564a971a2fa1b5860e934307cf7 # v5
         with:
-          release-type: simple
-          version-file: VERSION
+          config-file: release-please-config.json
+          manifest-file: .release-please-manifest.json