Reduce team review request noise from CODEOWNERS (#538)

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.

This:
- reduces team notification/review request noise for Dependabot and
  release automation PRs, given that other members on the team usually
  aren't going to look at such PRs anyway, and it only makes it harder
  for team members to see the review requests that are actually in
  their queue
- retains the "team awareness" benefit of having the team see
  notifications for all other non-automated PRs
- retains the UX benefit of having a reviewer be automatically selected

In the scenarios where either the primary repository maintainer is away,
or they are the person opening the PR (so can't request review from
themselves), other team members will still be able to approve the PR
with the changes I've made to the branch protection settings (unchecking
"Require Code Owner Review", leaving "require approvals" enabled which
can only be from teams with write access, which is only us. Plus for
bonus points, the option to only permit merging from members of our
team has also been enabled).

For supported syntax, see:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

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 repository maintainer instead for files that are
+# updated by release automation, to reduce team review request noise.
+CHANGELOG.md @joshwlewis