Add Mattermost changelog entry for version 7.1

• Release post: https://mattermost.com/blog/mattermost-v7-1-is-now-available/

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assigned to @hueser93

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Dear Tobias @frust45, I added a changelog entry for Mattermost to the technical documentation. Are you fine with this one?

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• 37a7bf8d - Add details to heading of Mattermost changelog entry

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approved this merge request

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merged

mentioned in commit 2d274282