Reduce team review request noise from CODEOWNERS (#37)

The `CODEOWNERS` file has been adjusted to request review from the
primary repository maintainer for high-traffic files that are
typically updated via automation, rather than requesting review from
the whole team.

For more information, see:
heroku/heroku-buildpack-ruby#1419

GUS-W-14941625.

.github/CODEOWNERS

@@ -1 +1,6 @@
+# Default to requesting pull request reviews from the Heroku Languages team.
 * @heroku/languages
+
+# However, request review from the language owner instead for files that are
+# updated by Dependabot, to reduce team review request noise.
+Gemfile.lock @schneems