Fix Mattermost notification JSON parsing error

Fixed "Expected property name or '}' in JSON at position 1" error in
Mattermost notification workflows by updating to modern GitHub Actions
output syntax.

Changes:
- Replace deprecated ::set-output with $GITHUB_OUTPUT
- Use PAYLOAD: |- multiline YAML syntax for proper JSON handling
- Applied to both pullrequest-mattermost.yaml and issues-mattermost.yaml
- Matches the working pattern in builds-mattermost.yaml

The deprecated ::set-output command doesn't properly handle JSON with
newlines and special characters. The new approach using $GITHUB_OUTPUT
and multiline YAML syntax preserves JSON structure correctly.

Author: KrisSimon

.github/workflows/issues-mattermost.yaml

@@ -62,9 +62,10 @@ jobs:
             }' > mattermost.json
       - name: Read Content
         id: getcontent
-        run: echo "::set-output name=payload::$(cat mattermost.json)"            
+        run: echo "payload=$(cat mattermost.json)" >> $GITHUB_OUTPUT
       - name: Send Mattermost Message
         uses: mattermost/action-mattermost-notify@master
         with:
           MATTERMOST_WEBHOOK_URL: ${{ secrets.WEBHOOK || secrets.MATTERMOST_WEBHOOK_URL }}
-          PAYLOAD: ${{ steps.getcontent.outputs.payload }}
+          PAYLOAD: |-
+            ${{ steps.getcontent.outputs.payload }}

.github/workflows/pullrequest-mattermost.yaml

@@ -42,9 +42,10 @@ jobs:
           cat mattermost.json
       - name: Read Content
         id: getcontent
-        run: echo "::set-output name=payload::$(cat mattermost.json)"          
+        run: echo "payload=$(cat mattermost.json)" >> $GITHUB_OUTPUT
       - name: Send Mattermost Message
         uses: mattermost/action-mattermost-notify@master
         with:
           MATTERMOST_WEBHOOK_URL: ${{ secrets.WEBHOOK || secrets.MATTERMOST_WEBHOOK_URL }}
-          PAYLOAD: ${{ steps.getcontent.outputs.payload }}
+          PAYLOAD: |-
+            ${{ steps.getcontent.outputs.payload }}