Potential breaking changes in PowerShell 7.6 Functions worker

[andystaples]

NOTE: PowerShell functions team is still investigating the impact of these issues. Please use caution around these scenarios when testing with PowerShell 7.6 and Functions.

From PS 7.5:

• ConvertTo-Json now serializes BigInteger as a number (#21000) (Thanks @jborean93!)

From PS 7.6:

• The Microsoft.PowerShell.ThreadJob replaces the ThreadJob module.
  • TBD: Will the Functions worker replace the bundled ThreadJob to match?

Positive changes:
Fix -Debug to not trigger the ShouldProcess prompt (#26081)

[andystaples]

Another TODO: double-check PowerShellManager's GetJobs reflection