fix(release): use stable versioning instead of prerelease

Both SDKs are at a stable 1.30.0, but the scaffolded release-please-config.json
still carried the prerelease strategy, which would cut 1.31.0-alpha.1 on the
first stock release-please run.

  versioning: prerelease -> default   (the field that adds the -alpha.N suffix)
  prerelease: true -> false           (marks the GitHub Release as a prerelease)

Both values verified against googleapis/release-please's published config
schema: versioning "defaults to `default`", prerelease is "create the GitHub
release as prerelease".

Hand-edited rather than configured: publish.release.prerelease: false in
stainless.yml does not reach this file. stlc scaffolds it once and preserves it
thereafter, and --rewrite-scaffold does not refresh it — with the file deleted
and --rewrite-scaffold passed, stlc did not recreate it at all. Preserved
scaffold is the documented place for hand-edits to survive.

Author: tode-rl

release-please-config.json

@@ -5,8 +5,8 @@
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
   "include-v-in-tag": true,
   "include-component-in-tag": false,
-  "versioning": "prerelease",
-  "prerelease": true,
+  "versioning": "default",
+  "prerelease": false,
   "bump-minor-pre-major": true,
   "bump-patch-for-minor-pre-major": false,
   "pull-request-header": "Automated Release PR",