Support Julia 1.6 for Turing and others

[gdalle]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Judging by the discussion with @devmotion in tpapp/LogDensityProblemsAD.jl#29 (comment), support for 1.6 is important in other parts of the ecosystem, even though SciML is willing to drop it sooner. The maintenance burden is very minimal in ADTypes.jl, so I'm adding it again.

Related discussions:

• Restructure sparse backends and replace subtyping by traits #40 (comment)
• Support Julia 1.6 (again) #48 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d4a014a) to head (9a7fa8d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           64        64           
=========================================
  Hits            64        64           

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