In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50063 we
introduced code for generating Markdown changelogs using the API, using
a custom template language. The compiler for this language would convert
a custom syntax into ERB, making sure arbitrary code execution isn't
possible; or so we thought. In
https://gitlab.com/gitlab-org/gitlab/-/issues/300224 we found a way to
bypass the template engine's harness, and run arbitrary Ruby code.

In response to this issue, I decided to investigate replacing the setup
with something more secure. We always planned on doing so when deemed
necessary, unfortunately that need arrived sooner than expected.

In this commit we replace the regex/ERB based setup with a parser built
using Parslet (http://kschiess.github.io/parslet/). Parslet makes it
pretty easy to write a parser, and was already an indirect dependency of
GitLab (through the license_finder Gem). This new parser doesn't allow
for arbitrary code execution, doesn't depend on ERB, and is less fragile
compared to the old setup. Templates are executed by walking and
evaluating the AST nodes the parser produces. While this won't break any
speed records, it's easy to maintain and understand, and fast enough for
our needs.

In this new setup there is a slight difference compared to the old
setup. In the old setup, expression tags on their own line don't add a
new line. So this:

    foo
    {% if something %}{% end %}
    bar

Compiles into this:

    foo
    bar

Getting this right using the Parslet parser proved difficult, so we took
a slightly different approach: any newline following an expression tag
(if, else, end, and each) consumes the newline that directly follows it
(if any). This requires only a small change in the template, is easy to
implement, and still intuitive to the user.

As part of this commit we also fix a small bug that would lead to empty
entries being included in the template, and add a note about using the
right YAML syntax to preserve newlines.

Fix timeout problems on test webhooks

See merge request gitlab-org/gitlab!52646

Clarify documentation for audit_json.log

See merge request gitlab-org/gitlab!52829

GraphQL: Add fields selection to export requirements mutation

See merge request gitlab-org/gitlab!52706

- Add new argument to GraphQL mutation
- Add permitted fields to export service
- Add specs

Fix bug, where the download email patches and plain diffs links did not download from the commit page

See merge request gitlab-org/gitlab!52899

Fix snippet repository storage move API when snippet does not exist

See merge request gitlab-org/gitlab!53211

Disable spellcheck on metrics dictionary

See merge request gitlab-org/gitlab!53308

This page can't be brought up to GitLab tone and style because it
is autogenerated by a script we can't easily change. Disable spell
checking on this page.

Fix opensearch for anonymous users

See merge request gitlab-org/gitlab!53056

Update question mark icon while checking pipeline status

See merge request gitlab-org/gitlab!52760

Apply new GitLab UI for badges in the project branch list

See merge request gitlab-org/gitlab!52868

Updated diffs notes app to use paginated notes

See merge request gitlab-org/gitlab!52089

BulkImports: always log the current pipeline

See merge request gitlab-org/gitlab!53257

Update docs review apps to use project token

See merge request gitlab-org/gitlab!53336

Merge branch '294266-parse-example-alert-payload-to-return-list-of-payload-alert-fields' into 'master'

Resolve "Parse example alert payload to return list of payload alert fields"

See merge request gitlab-org/gitlab!50823

Use gl-dropdown in pipeline manual actions

See merge request gitlab-org/gitlab!53223

Edited UI copy to comply with GitLab style

See merge request gitlab-org/gitlab!50676

Remove unused tree files

See merge request gitlab-org/gitlab!53188

Disable ETag caching on notes due to issues with pagination

See merge request gitlab-org/gitlab!52765

Add the job size to start message logging

See merge request gitlab-org/gitlab!53248

Remove retry on allow_local_requests_from_web_hooks_and_services

See merge request gitlab-org/gitlab!53140

Update Gitaly version

See merge request gitlab-org/gitlab!53339

Add dot to GraphQL descriptions of epic_type.rb

See merge request gitlab-org/gitlab!53228

Remove hardcoded values from projets api test

See merge request gitlab-org/gitlab!53126

Enable SSH key expiration enforcement

See merge request gitlab-org/gitlab!53035

Fix password reset link

See merge request gitlab-org/gitlab!53274

Apply new GitLab UI for buttons in tags page

See merge request gitlab-org/gitlab!52862

GitLab UI buttons and input style for project graph page

See merge request gitlab-org/gitlab!52864

Add descriptions to MergeRequestStateEnum

See merge request gitlab-org/gitlab!53082

Add missing values
Add descriptions to values
Update schema
Update documentation