Mark protobuf module as deprecated (#1778)

This moves the protobuf module from experimental to deprecated

Rationale:

* This is very seldom used, if used at all
* Not fully fleshed out (this really should be paired with a protoc-gen-pkl, which doesn't exist)
* There is a ProtoJSON wire format that we can already render to

stdlib/base.pkl

@@ -24,6 +24,7 @@ import "pkl:json"
 import "pkl:jsonnet"
 import "pkl:math"
 import "pkl:pklbinary"
+// noinspection Deprecated
 import "pkl:protobuf"
 import "pkl:xml"
 import "pkl:yaml"