Make static fields readonly

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/V2/OpenApiContactDeserializer.cs

@@ -36,7 +36,7 @@ internal static partial class OpenApiV2Deserializer
             },
         };
 
-        private static PatternFieldMap<OpenApiContact> _contactPatternFields = new PatternFieldMap<OpenApiContact>
+        private static readonly PatternFieldMap<OpenApiContact> _contactPatternFields = new PatternFieldMap<OpenApiContact>
         {
             {s => s.StartsWith("x-"), (o, p, n) => o.AddExtension(p, LoadExtension(p, n))}
         };

src/Microsoft.OpenApi.Readers/V2/OpenApiInfoDeserializer.cs

@@ -14,7 +14,7 @@ namespace Microsoft.OpenApi.Readers.V2
     /// </summary>
     internal static partial class OpenApiV2Deserializer
     {
-        private static FixedFieldMap<OpenApiInfo> _infoFixedFields = new FixedFieldMap<OpenApiInfo>
+        private static readonly FixedFieldMap<OpenApiInfo> _infoFixedFields = new FixedFieldMap<OpenApiInfo>
         {
             {
                 "title", (o, n) =>
@@ -54,7 +54,7 @@ internal static partial class OpenApiV2Deserializer
             }
         };
 
-        private static PatternFieldMap<OpenApiInfo> _infoPatternFields = new PatternFieldMap<OpenApiInfo>
+        private static readonly PatternFieldMap<OpenApiInfo> _infoPatternFields = new PatternFieldMap<OpenApiInfo>
         {
             {s => s.StartsWith("x-"), (o, p, n) => o.AddExtension(p, LoadExtension(p, n))}
         };

src/Microsoft.OpenApi.Readers/V2/OpenApiParameterDeserializer.cs

@@ -20,7 +20,7 @@ namespace Microsoft.OpenApi.Readers.V2
     internal static partial class OpenApiV2Deserializer
     {
         private static JsonSchemaBuilder _parameterJsonSchemaBuilder;
-        private static FixedFieldMap<OpenApiParameter> _parameterFixedFields =
+        private static readonly FixedFieldMap<OpenApiParameter> _parameterFixedFields =
             new FixedFieldMap<OpenApiParameter>
             {
                 {