drop support for Julia <v1.6

[jishnub]

It's unlikely that users of Julia v1.0 would want the latest versions of packages, so it makes sense to restrict further development to v1.6. In any case, this is meant to be merged along with the next breaking release, so that v0.8 may still receive backported changes, if necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (49bde1d) to head (c7051c8).
⚠️ Report is 36 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   94.98%   94.87%   -0.12%     
==========================================
  Files           7        7              
  Lines         798      819      +21     
==========================================
+ Hits          758      777      +19     
- Misses         40       42       +2     

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[jishnub]

On second thoughts, this isn't a breaking change, and someone who is using julia 1.0 is probably more concerned about stability rather than the latest versions of packages. I would suggest merging this, and moving on.

[dlfivefifty]

The convention is that dropping old versions of Julia is a breaking change

[jishnub]

It appears that colprac doesn't consider dropping support for older Julia versions as a breaking change, although it's a bit blurry for pre-1.0 packages. One needs to decide whether changes will be backported to v1.0, which I doubt