Use a custom regex for changelog tags

The tag format we use for gitlab-org/gitlab doesn't follow semantic
versioning; at least not as we expected. The -ee suffix we use for tags
is treated as the pre-release identifier, and when generating changelogs
we ignore pre-release tags. To solve this we need a custom tag regex
to ignore the -ee suffix.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1776 for
more information.

.gitlab/changelog_config.yml

@@ -36,3 +36,8 @@ template: |
   {% else %}
   No changes.
   {% end %}
+# The tag format for gitlab-org/gitlab is vX.Y.Z(-rcX)-ee. The -ee prefix would
+# be treated as a pre-release identifier, which can result in the wrong tag
+# being used as the starting point of a changelog commit range. The custom regex
+# here is used to ensure we find the correct tag.
+tag_regex: '^v(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)-ee$'