Lucene's gradle version can't cope with Java 14

If you have JDK 14 installed as your default java, then attempting to use gradle within the lucene-solr project can result in errors, particularly if you have other projects that use more recent gradle versions on the same machine.

```
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)
```