Skip failing jgit tests on Java 8

For some reason, smoke tests using jgit start failing with
"org/eclipse/jgit/api/errors/JGitInternalException has been compiled by
a more recent version of the Java Runtime (class file version 55.0)"
today. Before we figure out, disable them on Java 8 executer.

subprojects/smoke-test/src/smokeTest/groovy/org/gradle/smoketests/GradleGitPropertiesSmokeTest.groovy

@@ -16,6 +16,10 @@
 
 package org.gradle.smoketests
 
+import org.gradle.util.Requires
+import org.gradle.util.TestPrecondition
+
+@Requires(TestPrecondition.JDK11_OR_LATER)
 class GradleGitPropertiesSmokeTest extends AbstractPluginValidatingSmokeTest {
     @Override
     Map<String, Versions> getPluginsToValidate() {

subprojects/smoke-test/src/smokeTest/groovy/org/gradle/smoketests/JGitPluginSmokeTest.groovy

@@ -17,13 +17,15 @@
 package org.gradle.smoketests
 
 import org.gradle.integtests.fixtures.ToBeFixedForConfigurationCache
+import org.gradle.util.Requires
+import org.gradle.util.TestPrecondition
 import org.gradle.vcs.fixtures.GitFileRepository
 import spock.lang.Issue
 
 import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
 
+@Requires(TestPrecondition.JDK11_OR_LATER)
 class JGitPluginSmokeTest extends AbstractPluginValidatingSmokeTest {
-
     @Issue('https://plugins.gradle.org/plugin/org.ajoberstar.grgit')
     @ToBeFixedForConfigurationCache(because = "Gradle.buildFinished")
     def 'org.ajoberstar.grgit plugin'() {