Delegate parsing of targetFramework to dotnet CLI

[gukoff]

Issue describing the changes in this PR

Instead of parsing the .csproj file, determine the target framework by calling dotnet build -getproperty:TargetFramework.

It "works on my machine" (OSX, dotnet 8.0.201). I'm not aware if this solution has portability issues.

resolves #3565

Alternatives considered

I tried using new Project(path) instead of ProjectRootElement.Open(path) in hopes that new Project would find and import any external properties.

It didn't run. My .csproj file starts with <Project Sdk="Microsoft.NET.Sdk">, and calling new Project(csproj_content) throws exception The SDK 'Microsoft.NET.Sdk' specified could not be found.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• [?] My changes do not need to be backported to a previous version
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• I have added all required tests (Unit tests, E2E tests)