Rolling out `unified_diff_lines` feature flag

What

Remove the :unified_diff_lines feature flag. Rolling out this feature flag is going to be slightly different to normal as the feature flag is currently surrounded by a lot of tech debt.

Instead of enabling by default and then removing the flag in X milestone, we are going to wait for !40131 (merged) to hit GitLab.com, enable and test it out as much as possible. Once we are confident the change works as expected, we can look at removing all the associated tech debt code.

The tech debt involves:

  1. Removing the use of highlighted_diff_lines.
    • This is an old property that is being replaced (for now) with parallel_diff_lines
  2. Change all references of parallel_diff_lines to something like lines

The only issue I see with the above plan is that it is going to touch a lot of files 😕

Owners

  • Team: groupsource code
  • Most appropriate slack channel to reach out to: #g_create_source-code-fe
  • Best individual to reach out to: @iamphill

Expectations

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

  • gitlab-org/gitlab project
  • gitlab-org/gitlab-com groups
  • ...

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
  • Coordinate a time to enable the flag with #production and #g_delivery on slack.
  • 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 (more guidance when this is necessary in the dev docs) and in your team channel
  • Announce on the issue that the flag has been enabled
  • Remove feature flag and add changelog entry
  • After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

/cc @andr3