Disable the legacy Mixin Annotation Processor by default. (#1343)

src/main/java/net/fabricmc/loom/extension/MixinExtensionApiImpl.java

@@ -49,7 +49,7 @@ public abstract class MixinExtensionApiImpl implements MixinExtensionAPI {
 	public MixinExtensionApiImpl(Project project) {
 		this.project = Objects.requireNonNull(project);
 		this.useMixinAp = project.getObjects().property(Boolean.class)
-				.convention(true);
+				.convention(false);
 
 		this.refmapTargetNamespace = project.getObjects().property(String.class)
 				.convention(MappingsNamespace.INTERMEDIARY.toString());

src/test/groovy/net/fabricmc/loom/test/benchmark/FabricAPIBenchmark.groovy

@@ -48,10 +48,10 @@ class FabricAPIBenchmark implements GradleProjectTestTrait {
 				patch: "fabric_api"
 				)
 
-		if (!gradle.buildGradle.text.contains("loom.mixin.useLegacyMixinAp")) {
+		if (gradle.buildGradle.text.contains("loom.mixin.useLegacyMixinAp")) {
 			gradle.buildGradle << """
 				allprojects {
-					loom.mixin.useLegacyMixinAp = false
+					loom.mixin.useLegacyMixinAp = true
 				}
 				""".stripIndent()
 		}

src/test/groovy/net/fabricmc/loom/test/integration/FabricAPITest.groovy

@@ -50,10 +50,10 @@ class FabricAPITest extends Specification implements GradleProjectTestTrait {
 				)
 
 		// Disable the mixin ap if needed. Fabric API is a large enough test project to see if something breaks.
-		if (disableMixinAp) {
+		if (!disableMixinAp) {
 			gradle.buildGradle << """
 				allprojects {
-					loom.mixin.useLegacyMixinAp = false
+					loom.mixin.useLegacyMixinAp = true
 				}
 				""".stripIndent()
 		}

src/test/groovy/net/fabricmc/loom/test/integration/SimpleProjectTest.groovy

@@ -87,12 +87,12 @@ class SimpleProjectTest extends Specification implements GradleProjectTestTrait
 	}
 
 	@Unroll
-	def "remap mixins with tiny-remapper"() {
+	def "remap mixins with mixin AP"() {
 		setup:
 		def gradle = gradleProject(project: "simple", version: PRE_RELEASE_GRADLE)
 		gradle.buildGradle << """
 				allprojects {
-					loom.mixin.useLegacyMixinAp = false
+					loom.mixin.useLegacyMixinAp = true
 				}
 				""".stripIndent()