Update dependency org.grails:grails-gorm-testing-support to v3

Author: renovate[bot]

gradle.properties

@@ -30,7 +30,7 @@ fieldsVersion=3.0.0.RC1
 grailsVersion=6.0.0-SNAPSHOT
 micrometer.version=1.8.0
 scaffoldingVersion=4.1.0
-testingSupportVersion=2.6.1
+testingSupportVersion=3.0.0
 org.gradle.caching=true
 org.gradle.parallel=false
 org.gradle.daemon=true