Anim: Make experimental.use_animation_baklava read-only when non-exp

Make the RNA property `prefs.experimental.use_animation_baklava` read-only
when Blender was built without experimental features.

This reverts parts of b46e2e6300, which
completely removed the flag. That was a bit over-zealous, as there is still
Python code that checks it. Now it always stays `False` on non-experimental
builds.

Pull Request: https://projects.blender.org/blender/blender/pulls/128494

source/blender/makesrna/intern/rna_userdef.cc

@@ -7583,16 +7583,19 @@ static void rna_def_userdef_experimental(BlenderRNA *brna)
                            "work around invalid usercount handling in code that may lead to loss "
                            "of data due to wrongly detected unused data-blocks");
 
-#  ifdef WITH_ANIM_BAKLAVA
-  /* Only allow setting this to 'true' when actually built with Baklava. */
   prop = RNA_def_property(srna, "use_animation_baklava", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, nullptr, "use_animation_baklava", 1);
   RNA_def_property_ui_text(
       prop,
       "Multi-Slot Actions",
       "The new 'layered' Action can contain the animation for multiple data-blocks at once");
-  RNA_def_property_update(prop, 0, "rna_userdef_update");
+#  ifndef WITH_ANIM_BAKLAVA
+  /* Only allow setting this to 'true' when actually built with Baklava. Some of the Baklava code
+   * is not guarded with `WITH_ANIM_BAKLAVA`, but rather assumes that this flag is always 'false'
+   * then. */
+  RNA_def_property_clear_flag(prop, PROP_EDITABLE);
 #  endif
+  RNA_def_property_update(prop, 0, "rna_userdef_update");
 }
 
 static void rna_def_userdef_addon_collection(BlenderRNA *brna, PropertyRNA *cprop)