Increment minimum supported Julia version to 1.6?

[GordStephen]

Julia v1.6 is now an LTS release. PRAS is not compatible with the previous 1.0 LTS release (since threaded coroutines are only available in 1.3+). Furthermore, the stack-allocated views introduced in v1.5 present some nice opportunities for cleaner code without performance penalties (we'd used views in some hot loops in the past, but moved to passing full arrays plus indices because of performance issues). So it might be nice to require at least Julia v1.6 for PRAS v0.7, whenever that may be.