Update Mattermost to 5.30.1

What does this MR do?

The changelog for Mattermost 5.30 is available here. There are no major features in this release as far as I'm aware, but I can say that there are some important improvements to reduce lag that would make the app virtually unusable in certain circumstances.

There are no security fixes included in this release.

@twk3 @jasonblais @amyblais

Checklist

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened