Migrate Gitlab::Git::Repository#commit_count to Gitaly

Closes gitaly#355

See merge request !12688

Add variables to pipelines schedules

Closes #32568

See merge request !12372

Fix dashboard labels dropdown

Closes #34590

See merge request !12708

Native group milestones

See merge request !12280

Speed up `all_commit_shas` for new merge requests

See merge request !12706

Resolve "Inconsistent location of members page between groups and projects"

Closes #29893

See merge request !11560

N+1 problems on milestone page

Closes #34736

See merge request !12670

This makes the `Boards::CreateService` more reusable for EE

See merge request !12704

Follow-up from "Namespace license checks for multiple assignees"

Closes gitlab-ee#2839

See merge request !12701

Refactor several javascript files to ES module syntax

See merge request !12683

Fix ShaAttribute concern when there is no table

Closes #34798

See merge request !12705

For merge requests created after 9.4, we have a `merge_request_diff_commits`
table we can get all the SHAs from very quickly. We just need to exclude these
when we load from the legacy format, by ignoring diffs with no serialised
commits.

Once these have been migrated in the background, every MR will see this
improvement.

Add Group secret variables

Closes #12729

See merge request !12582

Remove option to disable Gitaly in Rails

See merge request !12677

Closes gitaly#355

When this is added to a new model, it would fail before the migrations were run
- including when trying to run migrations in production mode!

Update VERSION to 9.4.0-pre.

See merge request !12702

Extend MR tabs a bit to cover up the avatar holder and collapse icon on scroll

Closes #34153

See merge request !12589

Don't show auxiliary blob viewer for README when there is no wiki

Closes #34732

See merge request !12689

Upgrade GitLab Pages to v0.5.0

See merge request !12625

/reassign should only work when multiple assignees are available. And
since this is only the case for GitLab EE, remove it from CE.

This was picked using 517c97eb of gitlab-ee.

They are confusing.