chore: use explicit semver range to ignore setup-gradle v6

Replace '6.x' with '>= 6.0.0, < 7.0.0' to match Dependabot's
documented semver range format for github-actions ecosystem updates.

Author: curfew-marathon

.github/dependabot.yaml

@@ -50,7 +50,7 @@ updates:
       interval: "monthly"
     ignore:
       - dependency-name: "gradle/actions/setup-gradle"
-        versions: ["6.x"]
+        versions: [">= 6.0.0, < 7.0.0"]
     groups:
       dependencies:
         patterns: