Remove dependency on apiguardian-api in spring-test

The apiguardian-api dependency is once again a mandatory transitive
dependency of JUnit 5 since JUnit Jupiter 5.0.3.

spring-test/spring-test.gradle

@@ -35,7 +35,6 @@ dependencies {
 	optional("javax.websocket:javax.websocket-api:1.1")
 	optional("junit:junit:4.12")
 	optional("org.junit.jupiter:junit-jupiter-api:${junitJupiterVersion}")
-	optional("org.apiguardian:apiguardian-api:1.0.0")  // for compiling against JUnit 5 without warnings
 	optional("org.testng:testng:6.14.2")
 	optional("org.aspectj:aspectjweaver:${aspectjVersion}")
 	optional("org.codehaus.groovy:groovy-all:${groovyVersion}")