chore: reverts range index unavailable on netstandard2.0

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Reader/V2/OpenApiV2Deserializer.cs

@@ -88,7 +88,7 @@ private static IOpenApiExtension LoadExtension(string name, JsonNode node, Parsi
         private static (string, string?) GetReferenceIdAndExternalResource(string pointer)
         {
             var refSegments = pointer.Split('/');
-            var refId = refSegments[^1];
+            var refId = refSegments[refSegments.Length - 1];
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
 
             string? externalResource = isExternalResource ? $"{refSegments[0]}/{refSegments[1].TrimEnd('#')}" : null;

src/Microsoft.OpenApi/Reader/V3/OpenApiV3Deserializer.cs

@@ -155,7 +155,7 @@ private static (string, string?) GetReferenceIdAndExternalResource(string pointe
             JsonNodeHelper.ValidateReferencePointerFormat(pointer);
 
             var refSegments = pointer.Split('/');
-            var refId = refSegments[^1];
+            var refId = refSegments[refSegments.Length - 1];
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
 
             string? externalResource = null;

src/Microsoft.OpenApi/Reader/V31/OpenApiV31Deserializer.cs

@@ -161,7 +161,7 @@ private static (string, string?) GetReferenceIdAndExternalResource(string pointe
              * E.g. $ref: '#/components/schemas/pet'
              */
             var refSegments = pointer.Split('/');
-            string refId = !pointer.Contains('#') ? pointer : refSegments[^1];
+            string refId = !pointer.Contains('#') ? pointer : refSegments[refSegments.Length - 1];
 
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
             string? externalResource = null;

src/Microsoft.OpenApi/Reader/V32/OpenApiV32Deserializer.cs

@@ -161,7 +161,7 @@ private static (string, string?) GetReferenceIdAndExternalResource(string pointe
              * E.g. $ref: '#/components/schemas/pet'
              */
             var refSegments = pointer.Split('/');
-            string refId = !pointer.Contains('#') ? pointer : refSegments[^1];
+            string refId = !pointer.Contains('#') ? pointer : refSegments[refSegments.Length - 1];
 
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
             string? externalResource = null;