Pin mattermost-notify

Currently the action is always using the last published package on PyPi.
Pinning the is needed so changes to the code can be made, without switching everyone over to the new version immidiatly.

Ref.: DEVOPS-1269

mattermost-notify/action.yml

@@ -68,6 +68,7 @@ runs:
       with:
         python-version: "3.10"
         install: mattermost-notify
+        install-version: "24.11.2"
 
     - name: Set Channel and Webhook
       shell: bash