Prevent ApplicationSetting to cache nil value

Closes #39275

See merge request gitlab-org/gitlab-ce!14952

no CI replication

See merge request gitlab-org/gitlab-ee!3192

Add note about coming group milestone support

See merge request gitlab-org/gitlab-ee!3195

(EE-Port) Circuitbreaker backoff and retries

See merge request gitlab-org/gitlab-ee!3191

Remove problematic H1 from alpha_database.md

See merge request gitlab-org/gitlab-ee!3201

Simplify Geo installation docs: tracking DB is enabled by default now

Closes #3827

See merge request gitlab-org/gitlab-ee!3200

EE- remove new sidebar stuff

See merge request gitlab-org/gitlab-ee!3197

Port of "Remove Autosave class from global namespace" to EE

See merge request gitlab-org/gitlab-ee!3198

Closes #3827

[ci skip]

Rename the Alpha PG HA documentation to just the PG HA documentation

See merge request gitlab-org/gitlab-ee!3151

Refactor `have_http_status` into `have_gitlab_http_status` in the specs

See merge request gitlab-org/gitlab-ee!3183

Update using_docker_images and merge_requests docs

See merge request gitlab-org/gitlab-ee!3188

That way we can enable the circuitbreaker for just one host at a time.

Instead of only checking once within a timeout, check multiple times
within a timeout.

That means with a timeout of 30 seconds and 3 retries. Each try would
be allowed 20 seconds.

The circuitbreaker now has 2 failure modes:

- Backing off: This will raise the `Gitlab::Git::Storage::Failing`
  exception. Access to the shard is blocked temporarily.
- Circuit broken: This will raise the
  `Gitlab::Git::Storage::CircuitBroken` exception. Access to the shard
  will be blocked until the failures are reset.

Add loading button -- EE merge edition

See merge request gitlab-org/gitlab-ee!3165

Geo: Don't retry repositories or files until everything has been backfilled

Closes #3810

See merge request gitlab-org/gitlab-ee!3182

Reduce the quiet times between scheduler runs on Geo secondaries

Closes #3813

See merge request gitlab-org/gitlab-ee!3185

Encompasses designs from
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/23206#note_43355199
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/29985#note_30417407

[ci skip]

[ci skip]

CE upstream - Wednesday

See merge request gitlab-org/gitlab-ee!3167

Fix setting Geo sync capacity in the admin panel

See merge request gitlab-org/gitlab-ee!3184

Update Prometheus gem to fix problems with other Processes overwriting current metrics file

Closes #36714

See merge request gitlab-org/gitlab-ce!14827