Correct tag_format to tag_regex

The field is called "tag_regex", not "tag_format". This lead to some
Omnibus releases not having a changelog, as the API would use the
incorrect starting point for the range of commits to include.

.gitlab/changelog_config.yml

@@ -42,4 +42,4 @@ template: |
 #
 # This does not affect the commits included in the range, as no changelog
 # commits are introduced between the tag commits.
-tag_format: '^(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(\+(?P<pre>rc\d+))?((\.|\+)(?P<meta>ee\.\d+))?$'
+tag_regex: '^(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(\+(?P<pre>rc\d+))?((\.|\+)(?P<meta>ee\.\d+))?$'