chore: remove custom style

- Deleted the custom_style field from WorkspaceProfile and WorkspaceSetting messages in workspace_service.proto.
- Updated the corresponding Go files and gRPC service implementations to reflect the removal of custom_style.
- Removed references to custom_style in the API documentation and Swagger definitions.
- Adjusted the workspace setting migration logic to exclude custom_style.
- Updated README files to remove mentions of custom_style.
- Bumped protoc-gen-go and protoc-gen-go-grpc versions in generated files.

proto/store/workspace_setting.proto

@@ -23,7 +23,6 @@ message WorkspaceSetting {
     string license_key = 2;
     string instance_url = 3;
     bytes branding = 4;
-    string custom_style = 5;
   }
 
   message SecuritySetting {
@@ -56,8 +55,6 @@ enum WorkspaceSettingKey {
   WORKSPACE_SETTING_LICENSE_KEY = 10;
   // The secret session key used to encrypt session data.
   WORKSPACE_SETTING_SECRET_SESSION = 11;
-  // The custom style.
-  WORKSPACE_SETTING_CUSTOM_STYLE = 12;
   // The default visibility of shortcuts and collections.
   WORKSPACE_SETTING_DEFAULT_VISIBILITY = 13;
 }