Update appVersion of Mailroom chart during release
Summary
With gitlab-org/release-tools!749 (merged), version of mailroom used in gitlab-mailroom image gets updated to what is mentioned in GitLab's Gemfile during a release. However, the version specified in Mailroom chart's chart.yml file points to the GitLab version, not the mailroom version.
Proposal
Add method to version_fetcher script that will handle mailroom version bumping. This method should infer the version of mailroom from Gemfile.lock (similar to gitlab-org/release-tools!749 (merged)) and update the Chart.yml file.