Temporarily disable pragma SYSLIB0014 from CSharp CodeFix Verifiers

[carlossanlop]

Workaround to address the failure in the official builds:

/mnt/vss/_work/1/s/src/Test.Utilities/CSharpSecurityCodeFixVerifier`2+Test.cs(24,21): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014) [/mnt/vss/_work/1/s/src/Test.Utilities/Test.Utilities.csproj]
/mnt/vss/_work/1/s/src/Test.Utilities/CSharpSecurityCodeFixVerifier`2+Test.cs(29,21): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014) [/mnt/vss/_work/1/s/src/Test.Utilities/Test.Utilities.csproj]
/mnt/vss/_work/1/s/src/Test.Utilities/CSharpCodeFixVerifier`2+Test.cs(27,21): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014) [/mnt/vss/_work/1/s/src/Test.Utilities/Test.Utilities.csproj]
/mnt/vss/_work/1/s/src/Test.Utilities/CSharpCodeFixVerifier`2+Test.cs(32,21): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014) [/mnt/vss/_work/1/s/src/Test.Utilities/Test.Utilities.csproj]
...
D:\a\_work\1\s\src\NetAnalyzers\UnitTests\Microsoft.NetCore.Analyzers\Security\DataSetDataTableInWebSerializableObjectGraphTests.cs(93,13): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014)  [D:\a\_work\1\s\src\NetAnalyzers\UnitTests\Microsoft.CodeAnalysis.NetAnalyzers.UnitTests.csproj]
D:\a\_work\1\s\src\NetAnalyzers\UnitTests\Microsoft.NetCore.Analyzers\Security\DataSetDataTableInWebSerializableObjectGraphTests.cs(113,13): error SYSLIB0014: 'ServicePointManager' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead. Settings on ServicePointManager no longer affect SslStream or HttpClient.' (https://aka.ms/dotnet-warnings/SYSLIB0014) [D:\a\_work\1\s\src\NetAnalyzers\UnitTests\Microsoft.CodeAnalysis.NetAnalyzers.UnitTests.csproj]

The ideal fix would be to replace the functionality with the preferred new set of APIs.

This will be cherry-picked into the arcade deps flow for release/9.0.1xx:
#7386

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.49%. Comparing base (94955fd) to head (6bc04de).
Report is 243 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7387      +/-   ##
==========================================
+ Coverage   96.43%   96.49%   +0.06%     
==========================================
  Files        1412     1443      +31     
  Lines      336968   345885    +8917     
  Branches    11143    11374     +231     
==========================================
+ Hits       324946   333754    +8808     
- Misses       9218     9248      +30     
- Partials     2804     2883      +79