fix: use minor version bump for improvement

An improvement that's anything more than a bugfix cannot merely be a
patch release, per the semver spec.

config/conventionalcommits-release-rules.js

@@ -1,6 +1,6 @@
 module.exports = [
   {
     type: 'improvement',
-    release: 'patch',
+    release: 'minor',
   },
 ];