Drop Support for Windows PowerShell 5.1

[PlagueHO]

Question: Should support be dropped for PowerShell 5.1 and below?

This will simplify some code and eliminate the need to test on PS 5.1 as part of the CI/CD processes. Some changes proposed would prevent the module from working on PowerShell 5.1 so would need to be announced that it is no longer supported.

See #442

[Szeraax]

Implementing MaxRetires and RetryDelay like what Pwsh does would happen BEFORE the 429 error handling. There is no benefit to a PR for #442 in removing support for Windows Powershell. A PR for 442 will do a regular loop at lower priority than the 429 handler.