Enable Integrated error tracking

Integrated error tracking was disabled by default in #353639 (closed).

This issue tracks the roll out of integrated error tracking again.

This feature flag is used by gitlab-org/opstrace/opstrace#1728 (closed) in combination with use_click_house_database_for_error_tracking.

Tasks

1. Unify feature flags into gitlab_error_tracking

   1. • use_click_house_database_for_error_tracking - see #353956 (comment 1421550171).
   2. • integrated_error_tracking - see #353956 (comment 1421615435)

2. Check for gitlab_error_tracking additionally in Observability.enabled?

3. Disable observability_group_tab globally

4. Remove and cleanup observability_group_tab feature flag