Remove `update_legacy_slack_installations` feature flag

What does this MR do and why?

This MR removes the update_legacy_slack_installations feature flag to enable the feature

• #371810 (closed)
• #361732 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361732 (closed)