demote logging.warning about missing component layer to debug level

The current propagate_anchors implementation only finds a matching component layer that has the same layerId so only works for 'master' layers; to reduce the verbosity of the log for fonts that contain lots of special non-master (brace, bracket, etc) layers, for which we currently can't retrieve the corresponding component layer's anchors, for now we demote this from a warning to a debug message.
Note that the old anchor_propagation.py was only processing 'master' layers as well, so ignoring the special layers during anchor propagation is not a regression.

Lib/glyphsLib/builder/transformations/propagate_anchors.py

@@ -168,7 +168,7 @@ def anchors_traversing_components(
         # referenced have already been propagated
         anchors = get_component_layer_anchors(component, layer, glyphs, done_anchors)
         if anchors is None:
-            logger.warning(
+            logger.debug(
                 "could not get layer '%s' for component '%s' of glyph '%s'",
                 layer.layerId,
                 component.name,