POC: Add .NET 10 Target

[mtmk]

Minimum required changes to add .NET 10 target:

• X509Certificate2 constructors obsoleted (SYSLIB0057) — .NET 10 deprecates loading certificates via new
  X509Certificate2(...) and X509Certificate2Collection.Import(...). The replacement is X509CertificateLoader with methods
  like LoadPkcs12(), LoadPkcs12FromFile(), and LoadPkcs12CollectionFromFile().

• Span null comparison (CA2265) — .NET 10 flags comparing Span / ReadOnlySpan to null as an error, since null is
  implicitly converted to Span.Empty, making the check redundant.

• TFM-conditional package references — csproj conditions that matched specific TFMs (e.g. == 'net8.0') didn't account for
  net10.0, causing it to fall through to older package versions missing required APIs (keyed DI services).

TODO:

• Remove all .NET 6 conditionals
• Look into using new .NET 10 APIs e.g.
  • System.Threading.Lock
  • JsonSerializer.DeserializeAsync(PipeReader)

[mtmk]

copied to #1070