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

pkl-cli/src/main/kotlin/org/pkl/cli/CliEvaluatorOptions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,7 +51,7 @@ data class CliEvaluatorOptions(
    * - `"pcf"` (default)
    * - `"plist"`
    * - `"properties"`
-   * - `"textproto"`
+   * - `"textproto"` (deprecated in Pkl 0.33)
    * - `"xml"`
    * - `"yaml"`
    */