Fix javadoc when enable preview flag is added

changelog/@unreleased/pr-2338.v2.yml

@@ -0,0 +1,6 @@
+type: fix
+fix:
+  description: javadoc tasks are now properly configured when `--enable-preview` is
+    used
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/2338

gradle-baseline-java/src/main/groovy/com/palantir/baseline/plugins/javaversions/BaselineJavaVersion.java

@@ -124,6 +124,7 @@ public void execute(Task task) {
                         if (target.get().enablePreview()) {
                             // yes, javadoc truly takes a single-dash where everyone else takes a double dash
                             options.addBooleanOption("-enable-preview", true);
+                            options.setSource(target.get().javaLanguageVersion().toString());
                         }
                     }
                 });

gradle-baseline-java/src/test/groovy/com/palantir/baseline/BaselineJavaVersionIntegrationTest.groovy

@@ -147,6 +147,20 @@ class BaselineJavaVersionIntegrationTest extends IntegrationSpec {
         assertBytecodeVersion(compiledClass, JAVA_17_BYTECODE, ENABLE_PREVIEW_BYTECODE)
     }
 
+    def 'java 17 preview javadoc works'() {
+        when:
+        buildFile << '''
+        javaVersions {
+            libraryTarget = 11
+            distributionTarget = '17_PREVIEW'
+        }
+        '''.stripIndent(true)
+        file('src/main/java/Main.java') << java17PreviewCode
+
+        then:
+        runTasksSuccessfully('javadoc', '-i')
+    }
+
     def 'library target is used when no artifacts are published but project is overridden as a library'() {
         when:
         buildFile << '''