Add RuboCop cop for custom error classes

Closes #28770

See merge request !9573

Fixed long filename being under action buttons

Closes #26607

See merge request !9608

Remove the newrelic gem

Closes #12860

See merge request !9622

Removed top border from contribution calendar

Closes #28844

See merge request !9605

The frontend should use the v3 API for now

Closes #28863 and #28853

See merge request !9614

Visually display that expanded diff lines cant be commented on

See merge request !9465

Add view specs for CI/CD detailed status badge

Closes #25611

See merge request !9473

Disable the inheritance column of services in DisableInvalidServiceTemplates migration

Closes #28850

See merge request !9613

Replace create_new_project_button.png

See merge request !9403

Update docs to reflect Yarn support added in 8.17 not 9.0

See merge request !9590

Docs: Tags: Fix typo in parent_ids in example response

See merge request !9550

The `unless defined?(Service)` was useless since in production env,
models are eager loaded, thus we wouldn't disable the STI, resulting in
the following error:

The single-table inheritance mechanism failed to locate the subclass:
'GitlabCiService'. This error is raised because the column 'type' is
reserved for storing the class in case of inheritance. Please rename
this column if you didn't intend it to be used for storing the
inheritance class or overwrite Service.inheritance_column to use another
column for that
information./opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20170211073944_disable_invalid_service_templates.rb:11:in `up'
Signed-off-by: Rémy Coutable <remy@rymai.me>

Add note about how to use or escape variables inside other variables

See merge request !9601

The extra spacing at the top is caused the user-callout box that should be fixed in another merge request

Moved issue boards new issue form template

See merge request !9572

generalize the idea of a "unique internal user"

See merge request !9562

[ci skip]

Restructure pages docs

Closes #28688

See merge request !9527

#25437 Allow posting of just an emoji in comment; add /award slash command

Closes #25437

See merge request !9382

[ci skip]

From the Ruby style guide:

    # bad
    class FooError < StandardError
    end

    # okish
    class FooError < StandardError; end

    # good
    FooError = Class.new(StandardError)

This cop does that, but only for error classes (classes where the
superclass ends in 'Error'). We have empty controllers and models, which
are perfectly valid empty classes.

Closes #26607

Fix todos API endpoint application error

Closes #22951

See merge request !9351

Remove help from right dropdown

Closes #28837

See merge request !9594

But notice first.

See merge request !9364

[ci skip]

Closes #28844

Grammar improvement in gitlab flow intro doc

Closes #28696

See merge request !9552

Fix concurrent access on builds/register

See merge request !9600