Modernize JsonSchema.Deprecated storage

Use the C# 14 field keyword while preserving mutable-schema validation.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 69ae6b80-f573-41bd-8cf9-e0f858510bbe

Author: eiriktsarpalis

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchema.cs

@@ -79,8 +79,7 @@ public JsonSchema() { }
 
         public int? MaxLength { get; set { VerifyMutable(); field = value; } }
 
-        public bool? Deprecated { get => _deprecated; set { VerifyMutable(); _deprecated = value; } }
-        private bool? _deprecated;
+        public bool? Deprecated { get; set { VerifyMutable(); field = value; } }
 
         public JsonSchemaExporterContext? ExporterContext { get; set; }