Release GitLab for Slack notifications
What does this MR do and why?
This merge request removes the integration_slack_app_notifications
feature flag from the codebase.
The feature is GitLab.com only, so was fully released when the flag was toggled on prior. When the flag was toggled on, the related documentation change was merged !104639 (merged).
The commit includes a Changelog trailer to announce the release in our changelogs.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #381012 (closed)