Add the required gradle version to the documentation (#206)

Author: mmvpm

utbot-gradle/docs/utbot-gradle.md

@@ -250,4 +250,8 @@ Note that the internal gradle log information will also be shown.
 
 Also note that the standard way to configure the log level (using the `log4j2.xml`) does not work from gradle.
 
-[Read more about gradle log levels](https://docs.gradle.org/current/userguide/logging.html)
+[Read more about gradle log levels](https://docs.gradle.org/current/userguide/logging.html)
+
+### Requirements
+
+UTBot gradle plugin requires Gradle 6.8+