Fix JSON parsing error in Mattermost notification workflow

The workflow was using deprecated ::set-output syntax to pass JSON payload,
which caused 'Expected property name or } in JSON at position 1' errors.

Changes:
- Removed the intermediate 'Read Content' step that used ::set-output
- Changed from PAYLOAD to PAYLOAD_FILENAME parameter
- Now passes mattermost.json file directly to the action

This approach avoids JSON escaping issues when passing complex payloads
through GitHub Actions outputs. The mattermost-action-notify action
supports PAYLOAD_FILENAME for reading JSON from a file, which is more
reliable than passing JSON strings through output variables.

Fixes: https://github.com/uitsmijter/Documentation/actions/runs/19265980048

Author: KrisSimon

.github/workflows/builds-mattermost.yaml

@@ -56,11 +56,8 @@ jobs:
             --arg link "$LINK" \
             '{text: ("## \($icon) \($ref)\nStatus: \($status)\nin: [\($repo)](\($repo_url)) by: **\($author)**\n\n```text\n\($body)\n```\n----\n[Commit](\($link))")}' \
             > mattermost.json
-      - name: Read Content
-        id: getcontent
-        run: echo "::set-output name=payload::$(cat mattermost.json)"
       - name: Send Mattermost Message
         uses: mattermost/action-mattermost-notify@master
         with:
           MATTERMOST_WEBHOOK_URL: ${{ secrets.WEBHOOK }}
-          PAYLOAD: ${{ steps.getcontent.outputs.payload }}
+          PAYLOAD_FILENAME: mattermost.json