Mark deprecated fields with deprecated option in protobuf (#269)

This PR adds the protobuf `deprecated` option to properly mark
deprecated fields and message types in the dispatch API schema.

## Changes Made

1. **CategorySet message**: Added `option deprecated = true;` to mark
the entire message type as deprecated
2. **component_categories field**: Added `[deprecated = true]` to mark
the field as deprecated
3. **Release Notes**: Updated RELEASE_NOTES.md to document the
deprecation changes

## Before
```proto
// Deprecated: Use `CategoryTypeSet` instead
message CategorySet {
  // Set of component categories
  repeated frequenz.api.common.v1alpha8.microgrid.electrical_components
      .ElectricalComponentCategory categories = 1;
}

// Deprecated: Component categories
// Use `CategoryTypeSet` instead  
// In future versions, this field will be removed.
CategorySet component_categories = 2;
```

## After
```proto
// Deprecated: Use `CategoryTypeSet` instead
message CategorySet {
  option deprecated = true;
  // Set of component categories
  repeated frequenz.api.common.v1alpha8.microgrid.electrical_components
      .ElectricalComponentCategory categories = 1;
}

// Deprecated: Component categories
// Use `CategoryTypeSet` instead
// In future versions, this field will be removed.
CategorySet component_categories = 2 [deprecated = true];
```

## Benefits

- **Clear deprecation signaling**: Code generators will now properly
mark these elements as deprecated in generated client libraries
- **Developer warnings**: Developers using these deprecated fields will
receive appropriate deprecation warnings
- **API clarity**: Makes the deprecation status explicit in the protobuf
schema itself, not just in comments
- **Future-proof**: Prepares for eventual removal of these deprecated
elements

The changes follow the official protobuf specification for marking
deprecated elements and maintain full backward compatibility while
providing clear deprecation signals to API consumers.

Fixes #252.

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Author: Marenz

RELEASE_NOTES.md

@@ -14,6 +14,8 @@
 
 ## Upgrading
 
+- The `CategorySet` message and `component_categories` field in `TargetComponents` are now properly marked as deprecated in the protobuf schema using the `deprecated` option. Code generators will now emit deprecation warnings when these deprecated elements are used. Use `CategoryTypeSet` and `component_categories_types` instead.
+
 <!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
 
 ## New Features

proto/frequenz/api/dispatch/v1/dispatch.proto

@@ -391,6 +391,7 @@ message TargetComponents {
 
   // Deprecated: Use `CategoryTypeSet` instead
   message CategorySet {
+    option deprecated = true;
     // Set of component categories
     repeated frequenz.api.common.v1alpha8.microgrid.electrical_components
         .ElectricalComponentCategory categories = 1;
@@ -437,7 +438,7 @@ message TargetComponents {
     // Deprecated: Component categories
     // Use `CategoryTypeSet` instead
     // In future versions, this field will be removed.
-    CategorySet component_categories = 2;
+    CategorySet component_categories = 2 [deprecated = true];
 
     // Component categories with optional types
     CategoryTypeSet component_categories_types = 3;