[Feature flag] Enable user setting time preferences

What

Remove the :user_time_settings feature flag to enable by default the :

• time_format_in_24h profile setting
• time_display_relative profile setting Done in !65570 (merged)
• timezone profile setting Done in !65605 (merged)

Owners

• Team: Manage
• Most appropriate slack channel to reach out to: #g_manage
• Best individual to reach out to: Jeremy

Expectations

### What are we expecting to happen?

Users will be able to set their time preferences as per gitlab-ce!25381 and gitlab-ce#37900

What might happen if this goes wrong?

Time data across the application displays incorrectly

Roll Out Steps

• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry