Support JDK 20

The JDK 20 release does not recognize the `LAMBDA` feature enum constant.
Falling back to the `RECORDS` enum constant should guarantee support for the
next few JDK releases.

Author: Stephan202

example/build.gradle

@@ -7,7 +7,7 @@ configure([tasks.compileJava]) {
     options.release = 8
 
     javaCompiler = javaToolchains.compilerFor {
-        languageVersion = JavaLanguageVersion.of(19)
+        languageVersion = JavaLanguageVersion.of(20)
     }
 }
 

jabel-javac-plugin/src/main/java/com/github/bsideup/jabel/JabelCompilerPlugin.java

@@ -173,8 +173,13 @@ static void checkSourceLevel(
                 @Advice.Argument(value = 1, readOnly = false) Source.Feature feature
         ) {
             if (feature.allowedInSource(Source.JDK8)) {
-                //noinspection UnusedAssignment
-                feature = Source.Feature.LAMBDA;
+                if (Source.MIN.compareTo(Source.JDK8) < 0) {
+                  //noinspection UnusedAssignment
+                  feature = Source.Feature.LAMBDA;
+                } else {
+                  //noinspection UnusedAssignment
+                  feature = Source.Feature.RECORDS;
+                }
             }
         }
     }