Add changelog for feature flags encrypted tokens

We missed to add this in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29320.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/57918

changelogs/unreleased/57918-encrypt-feature-flags-tokens-changelog.yml

+---
+title: Add token_encrypted column to operations_feature_flags_clients table
+merge_request:
+author:
+type: other