* port retain build script from main

* Exclude retain-build.ps1 script from the telemetry check

…30801.2 (#13957)

[release/6.0] Update dependencies from dotnet/xharness

- small part of dotnet/dnceng#345
- bump Azure.Data.Tables to v12.8.1
- replace Microsoft.IdentityModel.Clients.ActiveDirectory with Microsoft.Identity.Client
  - part of dotnet/dnceng#629
- bump NuGet.* to v6.7.0
  - part of https://dev.azure.com/dnceng/internal/_workitems/edit/5128
  - part of https://dev.azure.com/dnceng/internal/_workitems/edit/5129
  - left both `$(NuGetVersioningVersion)` and `$(NuGetVersion)` to minimize disruption
    - see #12333 change in 'main' for a counter-example
  - react to deprecation of `ContentItemCollection.FindItemGroups(...)`
  - react to removal of `SignatureVerificationProviderFactory`
  - react to `VerifySignaturesResult` API changes (`Valid` to `IsValid`)
- bump NewtonSoft.Json to v13.0.3
- nits:
  - remove unused `$(...Version)` properties
  - remove a bit of dead code and unused `using`s in changed files

Co-authored-by: Viktor Hofer <[email protected]>

This is a slightly less expansive version of the main refactoring that removed sleet. I attempted to port that, but it's more extensive and I am more risk averse in 6.0/7.0. This basically removes the sleet reference, then NYIs the codepaths leading to it.