Temporarily removing use of ReadOnlySpan indexer.

[ahsonkhan]

Related to #25326

Removing the use of the ReadOnlySpan indexer to help the implementation change go through with the least disabling of tests.

This is a temporary workaround that is required to reduce the number of failing tests that would need to be disabled due to the change to ReadOnlySpan indexer to return ref readonly (in coreclr and in .NET Native)

cc @weshaggard, @stephentoub, @KrzysztofCwalina, @jkotas, @botaberg, @zamont

This change should be reverted once the change propagates

[ahsonkhan]

If there are no concerns, I would like to merge this, so we can get a new build available for .NET Native to consume.

[ahsonkhan]

@dotnet-bot test Linux x64 Release Build

[ahsonkhan]

Some System.Net.Security.Tests timed out for Ubuntu.1404.Amd64.Open
https://mc.dot.net/#/user/ahsonkhan/pr~2Fjenkins~2Fdotnet~2Fcorefx~2Fmaster~2F/test~2Ffunctional~2Fcli~2F/44c32786ac4add915227d7cbcdc65eb48ac46152/workItem/System.Net.Security.Tests

Message :
Assert.Throws() Failure
Expected: typeof(System.Security.Authentication.AuthenticationException)
Actual:   typeof(System.TimeoutException): VirtualNetwork: Timeout reading the next frame.
Stack Trace :
   at System.Net.Test.Common.VirtualNetwork.ReadFrame(Boolean server, Byte[]& buffer) in /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/Common/tests/System/Net/VirtualNetwork/VirtualNetwork.cs:line 39
   at System.Net.Test.Common.VirtualNetworkStream.Read(Byte[] buffer, Int32 offset, Int32 count) in /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/Common/tests/System/Net/VirtualNetwork/VirtualNetworkStream.cs:line 119
   at System.Threading.Tasks.Task`1.InnerInvoke() in /root/coreclr/src/mscorlib/src/System/Threading/Tasks/future.cs:line 610
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) in /root/coreclr/src/mscorlib/shared/System/Threading/ExecutionContext.cs:line 151
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot) in /root/coreclr/src/mscorlib/src/System/Threading/Tasks/Task.cs:line 2440
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.FixedSizeReader.<ReadPacketAsync>d__1.MoveNext() in /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/System.Net.Security/src/System/Net/FixedSizeReader.cs:line 56
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Security.SslState.EndProcessAuthentication(IAsyncResult result) in /mnt/j/workspace/dotnet_corefx/master/linux-TGroup_netcoreapp+CGroup_Release+AGroup_x64+TestOuter_false_prtest/src/System.Net.Security/src/System/Net/Security/SslState.cs:line 711
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization) in /root/coreclr/src/mscorlib/src/System/Threading/Tasks/FutureFactory.cs:line 533
--- End of stack trace from previous location where exception was thrown ---

cc @davidsh, @Priya, @wfurt