fix: escape master issue validation messages

Prevents auto @‘ing

Closes #2870

lib/workers/repository/error-config.js

@@ -10,7 +10,7 @@ async function raiseConfigWarningIssue(config, error) {
   }
   body += `Error type: ${error.validationError}\n`;
   if (error.validationMessage) {
-    body += `Message: ${error.validationMessage}\n`;
+    body += `Message: \`${error.validationMessage}\`\n`;
   }
   const pr = await platform.getBranchPr('renovate/configure');
   if (pr && pr.state && pr.state.startsWith('open')) {