Update MezaCoreExtensions.yml

Set Page Forms Version to the last version that supports MWM 1.39 (tag 5.9.1)

Author: ndc-rkevans

config/MezaCoreExtensions.yml

@@ -71,7 +71,7 @@ list:
   # PF - Allows users to add, edit and query data using forms.
   - name: PageForms
     repo: https://github.com/wikimedia/mediawiki-extensions-PageForms.git
-    version: master
+    version: tags/5.9.1
     config: |
       $wgPageFormsLinkAllRedLinksToForms = false;
       $wgPageFormsAutoeditNamespaces[] = NS_USER;