Update release tag regex

Author: Alex-Welsh

releasenotes/config.yaml

@@ -1,4 +1,4 @@
 ---
 # NOTE(upgrade): This needs to be updated to the current release's major version.
-release_tag_re: stackhpc/16\.\d+\.\d+\.\d
+release_tag_re: stackhpc/18\.\d+\.\d+\.\d
 ignore_null_merges: false