chore(deps): ignore toml v4+ in Dependabot to preserve Node.js 18 support (#13562)

toml v4.0.0 sets engines.node to ">=20", which conflicts with the
">=18.0" engine declared in packages/serverless/package.json. Add an
ignore entry mirroring the existing pattern used for other dependencies
that dropped Node 18 support.

Author: czubocha

.github/dependabot.yml

@@ -92,6 +92,9 @@ updates:
       - dependency-name: 'rimraf'
         # rimraf v6 drops support for Node.js 18
         versions: ['>= 6.0.0']
+      - dependency-name: 'toml'
+        # toml v4 drops support for Node.js 18
+        versions: ['>= 4.0.0']
 
   # Maintain Go dependencies
   - package-ecosystem: 'gomod'