Support JDK 14 and guava-28.2-jre

$ java -version
openjdk version "14" 2020-03-17
OpenJDK Runtime Environment (build 14+36-1461)
OpenJDK 64-Bit Server VM (build 14+36-1461, mixed mode, sharing)
$ ./gradlew build
Starting a Gradle Daemon, 1 busy and 1 incompatible Daemons could not be reused, use --status for details
java.lang.NoClassDefFoundError: Could not initialize class org.codehaus.groovy.vmplugin.v7.Java7
        at org.codehaus.groovy.vmplugin.VMPluginFactory.<clinit>(VMPluginFactory.java:43)
        at org.codehaus.groovy.reflection.GroovyClassValueFactory.<clinit>(GroovyClassValueFactory.java:35)
        at org.codehaus.groovy.reflection.ClassInfo.<clinit>(ClassInfo.java:107)
...
$ ./gradlew clean --stacktrace

FAILURE: Build failed with an exception.

* What went wrong:
Could not initialize class org.codehaus.groovy.runtime.InvokerHelper

* Try:
Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Exception is:
java.lang.NoClassDefFoundError: Could not initialize class org.codehaus.groovy.runtime.InvokerHelper
        at org.gradle.internal.extensibility.DefaultExtraPropertiesExtension.<init>(DefaultExtraPropertiesExtension.java:29)
        at org.gradle.internal.extensibility.DefaultConvention.<init>(DefaultConvention.java:49)
        at org.gradle.internal.extensibility.ExtensibleDynamicObject.<init>(ExtensibleDynamicObject.java:60)
...