csharp: use span-based ToBase64 in ByteString (#16976)

With .NET 5+, a copy can be avoided when the memory is not an array.

Closes #16976

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/16976 from verdie-g:main fd4f5aeee4
PiperOrigin-RevId: 638968774

csharp/src/Google.Protobuf/ByteString.cs

@@ -117,6 +117,9 @@ namespace Google.Protobuf
         /// <returns>A base64 representation of this <c>ByteString</c>.</returns>
         public string ToBase64()
         {
+#if NET5_0_OR_GREATER
+            return Convert.ToBase64String(bytes.Span);
+#else
             if (MemoryMarshal.TryGetArray(bytes, out ArraySegment<byte> segment))
             {
                 // Fast path. ByteString was created with an array, so pass the underlying array.
@@ -127,6 +130,7 @@ namespace Google.Protobuf
                 // Slow path. BytesString is not an array. Convert memory and pass result to ToBase64String.
                 return Convert.ToBase64String(bytes.ToArray());
             }
+#endif
         }
 
         /// <summary>