Fix potential security vulnerabilities.

[AraHaan]

Describe the bug

In my pr #1536 I have discovered that a few obsolete apis are used which in .NET Core is ignored which can result in some security vulnerabilities.

Of which include:

• the usage of WebClient, WebRequest, etc that were obsoleted for HttpClient.
• CER
• CAS

To reproduce

Currently the PR #1536 shows that to reproduce it, all you have to do is add net5.0 or net6.0 as shown in the PR to the SqlClient projects and then hit compile to get compile errors related to the deprecated apis and it saying not to use them as they can pose as an security issue.

Expected behavior

For the obsoleted apis to not be used as per Microsoft's recommendations from the documentations to each compile error that is the result from each of their usage.

Further technical details

Microsoft.Data.SqlClient version: version from repo in #1536
.NET target: .NET 6 (6.0.5)
SQL Server version: SQL Server 2019
Operating system: Windows 11

Additional context

I have made temporary changes to these in #1536, however I would like to split them out of that PR so they do not get blocked until .NET 6 is added to the CI.

[JRahnama]

@AraHaan are we talking about obsolete methods in net6? We know there are couple of them inside the driver, but since netcoreapp3.1 is still supported we cannot take them out or replace them. some of the replacements are not supported in the mentioned TFM.

[AraHaan]

In the PR I mentioned, I do currently special case .NET Core 3.1 (for now), however I do not know for sure if it uses an empty implementation for CAS and CER or if it uses the one from the .NET Framework in .NET Core 3.1.

[JRahnama]

@AraHaan These concerns are addressed in net 6 PR #1704. The obsolete values are only related to net6 and are still good in netcoreapp3.1. Closing the issue as the PR is under review.