Warn on missing subtype

Objective

Increase subtype label coverage by warning on MRs that do not have a subtype to allow teams to look at their output compared to planned issues.

Proposal

• Implement a danger warning when a MR does not have a sub type label
• Enable rule in gitlab-org/gitlab project

Relates to https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles/-/blob/master/lib/danger/rules/type_label/Dangerfile