Update Mattermost to 5.36.1

What does this MR do?

New features in Mattermost 5.36 include integration with Focalboard beta and a bunch of other UX improvements. It also includes a security fix which has been backported to previous Mattermost versions. The full changelog for it is here.

We're also getting close to finally moving to Mattermost 6.0, currently scheduled for our September release (so GitLab Omnibus would get it in October). There's a list of deprecated features which will be removed with Mattermost 6 here. I don't think any of them should affect GitLab users other than the new CLI tools and potentially the deprecation of the Mattermost Desktop App on Windows 7.

@twk3 @amyblais @jasonblais

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