Assign remaining integrations categories

What does this MR do and why?

Assign various remaining feature categories to owner groups documented in https://handbook.gitlab.com/handbook/product/cross-stage-features/#external-issue-trackers.

This follows the decision from Assign Product group DRIs for remaining integra... (gitlab-com/Product#14184 - closed).

Changes:

• Emails on push (2 files): :source_code_management
• Pipeline status emails (1 file): :continuous_integration
• Irker, Pushover (10 files): :team_planning
• Mattermost, (moved to !225209 (comment 3118689950))

References

• Similar to Assign Jira integration to team_planning (!224169 - merged)
• Similar to Assign Slack and Mattermost integrations to not... (!225209)
• Similar to Assign External Issue Tracker integrations to t... (!225213 - merged)
• Similar to Assign CI/CD integrations to continuous_integra... (!225230 - merged)
• Similar to Assign Prometheus integration to observability (!225232 - merged)
• Similar to Assign Packagist integration to package_registry (!225233 - merged)
• Similar to Assign Confluence integration to wiki (!225234 - merged)

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