Fix deprecated Gradle features warning (#3086)

This change fixes the following warning from the CI builds:

  Deprecated Gradle features were used in this build, making it incompatible with Gradle 10.

When warnings was turned on, Gradle was reporting:

  Properties should be assigned using the 'propName = value' syntax. Setting a property via the Gradle-generated 'propName value' or 'propName(value)' syntax in Groovy DSL has been deprecated.`:81`
  This is scheduled to be removed in Gradle 10.

  Properties should be assigned using the 'propName = value' syntax. Setting a property via the Gradle-generated 'propName value' or 'propName(value)' syntax in Groovy DSL has been deprecated.`:82`
  This is scheduled to be removed in Gradle 10.

Author: kahgoh

exercises/build.gradle

@@ -78,8 +78,8 @@ subprojects {
 
     // configuration of the linter
     checkstyle {
-      toolVersion '10.7.0'
-      configFile file("$rootDir/checkstyle.xml")
+      toolVersion = '10.7.0'
+      configFile = file("$rootDir/checkstyle.xml")
       sourceSets = [project.sourceSets.main, project.sourceSets.test]
     }