Update Mattermost to 5.38.2

Update to latest image from IB. NOTE: This update has security fixes.

Upstream changelog to verify no breaking changes/what is needed to switch images: https://docs.mattermost.com/administration/changelog.html

Definition of Done:

• Image version updated in chart/values.yaml
• Chart version bumped bb.x -> bb.y
• Changelog entry added, note the update to image, any additional updates
• Clean install tested and validated working (see things to test)
• Upgrade tested and validated working (install newest tagged release, send some chats/change settings, upgrade to your branch, validate things persisted, then run through things to test)
• Pipeline passing on package MR (update tests if the UI changes require it)
• Package MR merged and new MM release tagged
• BB MR opened, pipeline passing
• BB MR merged

Things to test:

• Test that signing up works and you can chat, validate that chats stick around and are persistent
• Test that logging out/in works with your account
• From the system console, go to file storage and test the connection, validate it succeeds. Also try to upload a profile picture.
• Install with dev SSO values (https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/master/chart/dev-sso-values.yaml#L82) and validate you can login via SSO
• Install with monitoring enabled and enterprise enabled, from the system console start a trial, go to prometheus and validate that the target for Mattermost shows as up