checkstyle: Bump default version to 9.3

Author: zml2008

indra-common/src/main/java/net/kyori/indra/internal/IndraExtensionImpl.java

@@ -51,7 +51,7 @@
 
 public class IndraExtensionImpl implements IndraExtension {
 
-  private static final String DEFAULT_CHECKSTYLE_VERSION = "8.42";
+  private static final String DEFAULT_CHECKSTYLE_VERSION = "9.3";
 
   private final Property<ContinuousIntegration> ci;
   private final Property<Issues> issues;