Update Mattermost to 5.34.4

What does this MR do?

Mattermost 5.34.4 contains a similar security fix to the one in Mattermost 5.35.3 and Mattermost 5.33.5 which we consider to be high severity enough to warrant backports.

It's been a while since we've had any backports to send to GitLab, so I'm hoping this branch is targeted correctly. Let me know if anything needs to be changed.

@twk3 @jasonblais @amyblais

Related issues

!5358 (merged) !5360 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

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

Closes #13 (closed)