Clean openApiGenerate output dir before generating (#2502)

This commit fixes a problem in the Solr build whereby files generated
from our OpenAPI spec could "leak" between branches and cause
compilation issues.

In short, generated SolrRequest implementations were not being
overwritten or cleaned up after switching branches.  This caused
compilation problems whenever the new branch didn't have other classes
(typically 'model' POJOs) that the generated file relied on.

This commit uses the openApiGenerate task's "cleanupOutput" option to
prevent this from happening.

solr/solrj/build.gradle

@@ -132,6 +132,7 @@ openApiGenerate {
   generateModelTests = false
   generateApiDocumentation = false
   generateModelDocumentation = false
+  cleanupOutput = true
   additionalProperties = ["modelPackage": "org.apache.solr.client.api.model"]
 }