Set default max Gradle version to '7.0'.

^KT-45744 In Progress

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/testbase/KGPBaseTest.kt

@@ -109,7 +109,7 @@ abstract class KGPBaseTest {
     @Retention(AnnotationRetention.RUNTIME)
     annotation class GradleTestVersions(
         val minVersion: String = minSupportedGradleVersion,
-        val maxVersion: String = "7.0-milestone-3",
+        val maxVersion: String = "7.0",
         val additionalVersions: Array<String> = []
     )
 }