Update registry1.dso.mil/ironbank/opensource/mattermost/mattermost Docker tag to v11.2.1

This MR contains the following updates:

Package	Update	Change
registry1.dso.mil/ironbank/opensource/mattermost/mattermost (source)	minor	11.1.1 -> 11.2.1

Complete MR checklist

Assignee

• Followed upgrade instructions outlined in docs/DEVELOPMENT_MAINTENANCE.md
• Update Docs with new/updated steps as needed
• Tested and Validated Changes made with supporting info like logs or screenshots from test pipelines

Add supporting info below

One link check error is still a live link, the other is in our dev maintenance doc and referring to the Mattermost url.

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• Tested and Validated changes

Linked Issue

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Upgrade Notices

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