Change deprecated Bazel single file attr param

This removes the need for --incompatible_disable_deprecated_attr_params
when using Protobuf

protobuf.bzl

diff --git a/protobuf.bzl b/protobuf.bzl
index 67d61d7..ae391aa 100644
--- a/protobuf.bzl
+++ b/protobuf.bzl
@@ -178,7 +178,7 @@
         "protoc": attr.label(
             cfg = "host",
             executable = True,
-            single_file = True,
+            allow_single_file = True,
             mandatory = True,
         ),
         "plugin": attr.label(