Remove Conditional Compilation and Runtime Checks for v3 or v4

[liamjpeters]

PR Summary

The minimum PS version the module can be loaded in is 5.1 (since e849068).

All of the conditional compilation checks around PSV3 and PSV4 can be removed/simplified.

Similarly the runtime checks for anything less than major version 5 can be simplified.

There are probably still some functions acting as compat shims to bridge functionality missing in 3 or 4 which could also be replaced.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
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[bergmeister]

Yes, when we upped minimum version to 5 in last release, we left code deliberately as-is in case there was feedback to pushback on that and allow people to . Whilst it has been noticed, I think we're ready to drop the code now as I'd like a new release in the next weeks or months (so will also go through other PRs). Thanks for taking the time :-)