Remove old mkVersion

[zlonast]

We don't have any testing for such old versions of ghc. Since this code isn't used, I assume it doesn't work, but it's difficult to confirm otherwise.

We just dropped everything below 8.0.0

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Template Α: This PR modifies behaviour or interface

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[zlonast]

Should we drop compilers below 8.6? Or 8.8?

[ffaf1]

This does modify behaviour, does it not?

[zlonast]

@ffaf1 It seems to me that if I can't compile the code with an old compiler, then this doesn't count as a change in behavior.

[ffaf1]

@ffaf1 It seems to me that if I can't compile the code with an old compiler, then this doesn't count as a change in behavior.

Makes sense!