Test both PostgreSQL and MySQL for the win

See merge request !1646

Make handler metric params more explicit

See merge request !1696

Refactor add_users method for project and group [EE]

See merge request !1697

This lets us track how many incoming emails a GitLab instance is processing, by
email type (handler) and by project (where applicable).

Remove duplicated tests

See merge request !1687

[ci skip]

[ci skip]

Docs for canary deployments vol 2

Closes gitlab-ce#31270

See merge request !1704

[ci skip]

Replace service_desk_enabled.png

See merge request !1701

Docs for resolvable discussions

See merge request !1700

Check if incoming emails and email key are available for service desk

Closes #2160

See merge request !1690

- Move to dicsussions/index.md
- Bring back previous path with a link to new location
- Reorder sections

Remove period from burndown charts warning help link

See merge request !1698

Port of environments-vue-3 to EE

See merge request !1693

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Simplify service desk project email

Closes #2029

See merge request !1668

Add warning when burndown data is not accurate + docs

Closes #2151

See merge request !1650

Fix specs and email address regex

Port of 31189-pipelines-error-state to EE

See merge request !1692

Add index to approvals.merge_request_id

Closes #2229

See merge request !1695

Show canaries first

Closes #2216

See merge request !1685