Change virtuals in abstract classes to abstract.

[brentschmaltz]

We added methods to an abstract class in a minor release and needed to mark them virtual and throw NotImplementedException. We should take the breaking compile change in the 8.x major release. TokenHandler https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/TokenHandler.cs#L66 https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/TokenHandler.cs#L76 https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/TokenHandler.cs#L85 SignatureProvider https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/SignatureProvider.cs#L105 https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/SignatureProvider.cs#L118 https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/55cc10ea53503b129afd55734ad9e9dd8203b339/src/Microsoft.IdentityModel.Tokens/SignatureProvider.cs#L150