Update the upgrade guide

See merge request gitlab-org/gitlab!46763

- Move relevant information from the maintenance doc
- Rearrange sections
- Merge the specific 12.0 section to the major upgrade section

Merge branch '13020-add-a-more-user-friendly-way-to-select-namespaces-in-jira-connect-app' into 'master'

Add Webpack, Vue and GitLab UI to Jira Connect app

See merge request gitlab-org/gitlab!45970

- Create a separate JS bundle for jira_connect
- Add ESLint

Generalize burnup chart service for other metrics

See merge request gitlab-org/gitlab!45121

Convert Jest tests to use VTU in 'ee/spec/frontend/sidebar'

See merge request gitlab-org/gitlab!43824

Instead of Promises.
Make sure all of them use `wrapper.vm` instead of using
`Vue`.
Make sure wrapper is set to null after each test.

Remove FF toggle logic from test

See merge request gitlab-org/gitlab!46487

Fix log lines being cut-off on failed job tab

See merge request gitlab-org/gitlab!46885

Renames TimeboxBurnupChartService to TimeboxReportService since this
will also be used to compute other statistics.

This also moves the burnup time series in GraphQL under a `report`
field.

Add username search to invite members modal

See merge request gitlab-org/gitlab!42398

Add frontend docs for dompurify

See merge request gitlab-org/gitlab!45502

Edit new alert HTTP endpoint

See merge request gitlab-org/gitlab!46697

Add support for alert HTTP
create supported via GraphQL

doc: Mention allowed prefixes in commit titles

See merge request gitlab-org/gitlab!46965

Signed-off-by: Rémy Coutable <remy@rymai.me>

Fixed layout of manual patch instructions

See merge request gitlab-org/gitlab!46932

Update details on secure templates with MR pipelines

See merge request gitlab-org/gitlab!46950

Document the merge requests deployment filters

See merge request gitlab-org/gitlab!46343

Document rspec metadata used in e2e tests

See merge request gitlab-org/gitlab!45879

Use loading prop for design upload button

See merge request gitlab-org/gitlab!46292

Changed label text for links from uppercase to lowercase to match title of page being linked to

See merge request gitlab-org/gitlab!45824

Ensure that pages metadata exists when updating it

See merge request gitlab-org/gitlab!46669

Add Migration Bot documentation

See merge request gitlab-org/gitlab!44276

- Move doc to dev/
- Reorg doc content
- Add "user types" to user/index.md
- Link from user/index.md

Display submodules in MR tree and file header

See merge request gitlab-org/gitlab!46840

Add geo secondary web user usage ping

See merge request gitlab-org/gitlab!46278

Avoid N+1 queries in JiraConnect::client

See merge request gitlab-org/gitlab!46783

We expose a user notes count for each MR. Counting
user notes executes an extra SQL query for each MR in the list.
This commit loads all counts in one query and passes it to the
serializer as an option. If the option is not available, it will
fall back to execute a count query.

This fix is a bit paranoid: pages_metadata was added in 12.*
and created via background migration.

Now it should exists for every project. (this is true for .com)

But if users just upgrade to 13.* after a long wait,
then background migration may still be in-progress, so we want
to ensure that metadata exist before we update it with
deployment_id

Remove target="_blank" from pipeline status widget

See merge request gitlab-org/gitlab!46893

Since the code changed is in EE as well.

Replace deprecated classes with utility ones

See merge request gitlab-org/gitlab!46911

Update stop environment modal to gl-modal

See merge request gitlab-org/gitlab!46469