Add a Directory.Package.props file to enable central package management.

This has 2 main advantages:
- Changes to versions only needs to be done 1 file (.csproj files are unchanged for NuGet updates)
- All projects use the same NuGet package version.

Enable central NuGet package version management

When restoring Nuget packages, dotnet restore will report the following warnings:

warning NU1902: Package 'BouncyCastle.Cryptography' 2.2.1 has a known moderate severity vulnerability, GHSA-8xfc-gm6g-vgpv
warning NU1903: Package 'MimeKit' 4.3.0 has a known high severity vulnerability, GHSA-gmc6-fwg3-75m5
warning NU1903: Package 'System.Formats.Asn1' 8.0.0 has a known high severity vulnerability, GHSA-447r-wph3-92pm
warning NU1903: Package 'System.Formats.Asn1' 7.0.0 has a known high severity vulnerability, GHSA-447r-wph3-92pm
warning NU1903: Package 'Microsoft.Extensions.Caching.Memory' 8.0.0 has a known high severity vulnerability, GHSA-qj66-m88j-hmgj
warning NU1903: Package 'NuGet.Common' 6.3.1 has a known high severity vulnerability, GHSA-6qmf-mmc7-6c2p
warning NU1903: Package 'NuGet.Protocol' 6.3.1 has a known high severity vulnerability, GHSA-6qmf-mmc7-6c2p
warning NU1903: Package 'System.Text.Json' 8.0.0 has a known high severity vulnerability, GHSA-8g4q-xg66-9fp4
warning NU1904: Package 'NuGet.Packaging' 6.3.1 has a known critical severity vulnerability, GHSA-68w7-72jg-6qpp
warning NU1903: Package 'System.Net.Http' 4.3.0 has a known high severity vulnerability, GHSA-7jgj-8wvc-jh57
warning NU1903: Package 'System.Text.RegularExpressions' 4.3.0 has a known high severity vulnerability, GHSA-cmhx-cq75-c4mj

Reporting vulnerability warnings is a recent new feature of NuGet functionality. Updating the used NuGet packages to the latest offical released versions has resolved these warnings.
NuGet packages that require .NET 9.0 are updated to the latest 8.x versions as updating TargetFramework to .NET 9.0 is considered out of scope of this PR.

Remarks:
 - System.Text.Json is a transient package which requires an explicit PackageVersion as the actual package Microsoft.VisualStudio.Web.CodeGeneration.Design cannot be updated as it requires .NET 9.0.
 - Microsoft.Extensions.PlatformAbstractions is a legacy package and not longer maintained. The source files that had a reference to these packages has been removed. The actual code/test inside these files was not used.

Reference:
Executing: dotnet nuget why "DasBlog All.sln" System.Text.Json
  [net8.0]
   └─ DasBlog.Web (v8.0.0)
      └─ Microsoft.VisualStudio.Web.CodeGeneration.Design (v8.0.6)
         ├─ Microsoft.DotNet.Scaffolding.Shared (v8.0.6)
            └─ Microsoft.CodeAnalysis.CSharp.Features (v4.8.0)
               └─ Microsoft.CodeAnalysis.Features (v4.8.0)
                  └─ System.Text.Json (v7.0.3)  <= Has vulnerability

Executing: dotnet nuget why "DasBlog All.sln" System.Net.Http
  [net8.0]
   └─ Microsoft.Extensions.PlatformAbstractions (v1.1.0) <= This package has been deprecated as it is legacy and is no longer maintained. (aspnet/Announcements#237)
      └─ NETStandard.Library (v1.6.1)
         └─ System.Net.Http (v4.3.0)  <= Has vulnerability

Executing: dotnet nuget why "DasBlog All.sln" System.Text.RegularExpressions
[net8.0]
   └─ Microsoft.Extensions.PlatformAbstractions (v1.1.0)  <= This package has been deprecated as it is legacy and is no longer maintained. (aspnet/Announcements#237)
      └─ NETStandard.Library (v1.6.1)
         ├─ System.Text.RegularExpressions (v4.3.0)
         ├─ System.Xml.ReaderWriter (v4.3.0)
         │  └─ System.Text.RegularExpressions (v4.3.0) <= Has vulnerability
         └─ System.Xml.XDocument (v4.3.0)
            └─ System.Xml.ReaderWriter (v4.3.0)
               └─ System.Text.RegularExpressions (v4.3.0)  <= Has vulnerability

…nings

Resolve NuGet warnings NU1902 and NU1903 by updating NuGet packages