drop support for pre-1.6 julia

[araujoms]

I think it's worth dropping support for these ancient versions in order to remove a couple of switches from the source code. I tested with Julia 1.0, it doesn't even run. With Julia 1.6 the tests pass.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.29%. Comparing base (2ef0305) to head (50eaf0c).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/type/show.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
+ Coverage   49.20%   49.29%   +0.08%     
==========================================
  Files          63       63              
  Lines        3410     3404       -6     
==========================================
  Hits         1678     1678              
+ Misses       1732     1726       -6     

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