Add apiguardian compileOnlyApi to fix Gradle build warnings

Gradle build on CI otherwise will warn about
"unknown enum constant Status.STABLE,  reason: class file for
org.apiguardian.api.API$Status not found"

build.gradle.kts

@@ -59,6 +59,7 @@ dependencies {
     // ↓ Classpath resource for MigrateTestNg* recipes
     testRuntimeOnly("org.testng:testng:7.5.1")  // 7.5.x is the last Java 8 compatible version: https://github.com/testng-team/testng/issues/2775
     runtimeOnly("org.junit.jupiter:junit-jupiter-api:latest.release")
+    testImplementation("org.apiguardian:apiguardian-api:latest.release")
 
     // ↓ to allow using testing recipes in our recipe list
     testRuntimeOnly("org.openrewrite.recipe:rewrite-testing-frameworks:latest.release") {