Assign Slack and Mattermost integrations to notifications

What does this MR do and why?

Change attribution of feature category from integrations to notifications.

This follows the decision from Assign Product group DRIs for remaining integra... (gitlab-com/Product#14184 - closed), documented in https://handbook.gitlab.com/handbook/product/cross-stage-features/#external-issue-trackers.

References

Similar to Assign Jira integration to team_planning (!224169 - merged)

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