Turn on --experimental_starlark_config_transitions

RELNOTES: Turn on --experimental_starlark_config_transitions by default for starlark transitions (see https://docs.bazel.build/versions/master/skylark/config.html#user-defined-transitions for more info)

SKIP_CI=blaze is borked
PiperOrigin-RevId: 250776283

src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java

@@ -140,11 +140,9 @@ public class StarlarkSemanticsOptions extends OptionsBase implements Serializabl
               + "debugging.")
   public boolean experimentalPlatformsApi;
 
-  // TODO(cparsons): Resolve and finalize the transition() API. The transition implementation
-  // function should accept two mandatory parameters, 'settings' and 'attr'.
   @Option(
       name = "experimental_starlark_config_transitions",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {OptionMetadataTag.EXPERIMENTAL},

src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java

@@ -225,7 +225,7 @@ public static Builder builderWithDefaults() {
           .experimentalGoogleLegacyApi(false)
           .experimentalJavaCommonCreateProviderEnabledPackages(ImmutableList.of())
           .experimentalPlatformsApi(false)
-          .experimentalStarlarkConfigTransitions(false)
+          .experimentalStarlarkConfigTransitions(true)
           .experimentalStarlarkUnusedInputsList(false)
           .incompatibleBzlDisallowLoadAfterStatement(true)
           .incompatibleDepsetIsNotIterable(false)