Migrate to modern Starlark linking api.

See https://github.com/bazelbuild/bazel/issues/10860.

bazel/upb_proto_library.bzl

@@ -183,10 +183,7 @@ def _upb_proto_rule_impl(ctx):
         fail("proto_library rule must generate _UpbWrappedCcInfo or " +
              "_UpbDefsWrappedCcInfo (aspect should have handled this).")
 
-    if type(cc_info.linking_context.libraries_to_link) == "list":
+    lib = cc_info.linking_context.linker_inputs.to_list()[0].libraries[0]
-        lib = cc_info.linking_context.libraries_to_link[0]
-    else:
-        lib = cc_info.linking_context.libraries_to_link.to_list()[0]
     files = _filter_none([
         lib.static_library,
         lib.pic_static_library,