Improved map-to-any casts based on fixed typings of google-protobuf

[renkei]

I'm not sure if it's worth but this PR changes the map-to-any casts in the generated code from

(pb_1.Map as any).deserializeBinary(message.map_field, reader, ...)

to

pb_1.Map.deserializeBinary(message.map_field as any, reader, ...)

because static Map.deserializeBinary(...) is finally available in @types/google-protobuf. Nevertheless, another cast for the first argument is still required because a protobuf Map is expected but an ES6 Map is passed.

[renkei]

I've updated the PR to an even newer @types/google-protobuf version 3.15.5 with additional fixes, see #55084 for details.

[thesayyn]

Thank you so much for working on this. I will get this released ASAP.