and simplify ProjectCiCdSetting#set_default_git_depth

We need to stub default_git_depth and default_git_depth= because some
old migrations specs try to create a record using schema before that
column was introduced.

If the origin project has no default_git_depth set (i.e. nil) set the
fork's default_git_depth to 0

Introduce default_git_depth in project's CI/CD settings and set it to
50. Use it if there is no GIT_DEPTH variable specified. Apply this
default only to newly created projects and keep it nil for old ones
in order to not break pipelines that rely on non-shallow clones.

default_git_depth can be updated from CI/CD Settings in the UI, must be
either nil or integer between 0 and 1000 (incl).

Inherit default_git_depth from the origin project when forking projects.

MR pipelines are run on a MR ref (refs/merge-requests/:iid/merge) and it
contains unique commit (i.e. merge commit) which doesn't exist in the
other branch/tags refs. We need to add it cause otherwise it may break
pipelines for old projects that have already enabled Pipelines for merge
results and have git depth 0.

Document new default_git_depth project CI/CD setting

Simplify notification check for participating

See merge request gitlab-org/gitlab-ce!29188

Backport of 10854-elasticsearch_avoid_db

See merge request gitlab-org/gitlab-ce!28579

Edit "SAML SSO" for SSOT

See merge request gitlab-org/gitlab-ce!29248

Edit Group's "Roadmap" for SSOT

See merge request gitlab-org/gitlab-ce!29245

Resolve "Inline validation for user's name and username length"

Closes #60323

See merge request gitlab-org/gitlab-ce!28095

Fix markdown for HA topic

See merge request gitlab-org/gitlab-ce!29244

Resolve "Default Web IDE's merge request checkbox to true"

Closes #60034

See merge request gitlab-org/gitlab-ce!28665

Introduce service for merge request pipeline creation

See merge request gitlab-org/gitlab-ce!28781

CE Backport: Show tree within Epic containing child Epics and Issues

See merge request gitlab-org/gitlab-ce!28787

Edit "Insights" content to meet SSOT guidelines

See merge request gitlab-org/gitlab-ce!29238

Edited "Epics" to comply with SSOT

See merge request gitlab-org/gitlab-ce!29236

Edited "Custom group-level project templates" for SSOT

See merge request gitlab-org/gitlab-ce!29235

Use the selected time window for metrics dashboard

See merge request gitlab-org/gitlab-ce!29152

Add note on weekly updates of dast Docker image

See merge request gitlab-org/gitlab-ce!29126

Allow BetterErrors REPL to be enabled

See merge request gitlab-org/gitlab-ce!29034

SSoT audit fixes for some topics in git section

Closes #61512

See merge request gitlab-org/gitlab-ce!28966

Implements part of the single source of truth audit
fixes for the topics/git section.

doc: add git mr command promotion

See merge request gitlab-org/gitlab-ce!28860

Update project security dashboard documentation

See merge request gitlab-org/gitlab-ce!28681

Avoid setting Gitlab::Session on sessionless requests and Git HTTP

See merge request gitlab-org/gitlab-ce!29146

CE backport for gitlab-ee!13389 (Support storing design blobs using LFS)

See merge request gitlab-org/gitlab-ce!29101

Docs: Tweak the wording regarding artifact completion

Closes #56470

See merge request gitlab-org/gitlab-ce!28880

[CE] Retry the first login page validation

Closes gitlab-org/quality/nightly#103

See merge request gitlab-org/gitlab-ce!29100