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

docs/modules/pkl-cli/pages/index.adoc

@@ -307,7 +307,7 @@ The default output renderer for a module supports the following formats:
 * `pcf`
 * `plist`
 * `properties`
-* `textproto`
+* `textproto` (deprecated in Pkl 0.33)
 * `xml`
 * `yaml`
 

docs/modules/pkl-gradle/pages/index.adoc

@@ -152,7 +152,7 @@ The default output renderer for a module supports the following formats:
 * `"pcf"`
 * `"plist"`
 * `"properties"`
-* `"textproto"`
+* `"textproto"` (deprecated in Pkl 0.33)
 * `"xml"`
 * `"yaml"`
 ====