Change the global.json parsing in Fake.Dotnet.Cli to use System.Text.Json instead of Newtonsoft.Json

[Numpsy]

Description

refs comments in #2825 about using System.Text.Json rather than Newtonsoft.Json

TODO

• The .NET 6.0(+) builds can use the inbox version of System.Text.Json, but the .NET Standard 2.0 build needs the NuGet package. As it stands, the package reference is transitive rather than direct.
• The code aorund here is called indirectly by the unit tests for Fake.Dotnet.Cli but I don't think there is anything direct for the global.json parsing (maybe there could be, to test things like json comment handling or issues like [REGRESSION] FAKE 6.1.0 fails with dotnet preview versions #2803)

[github-actions]

Test Results

    9 files  ±0      9 suites  ±0   25m 39s ⏱️ - 27m 25s
  439 tests ±0    439 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 251 runs  ±0  1 251 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 00e887e. ± Comparison against base commit 10f0b21.

♻️ This comment has been updated with latest results.