CI improvements

[ArnoStrouwen]

Makes CI more uniform with rest of SciML. Removes testing for 1.6, since rest of SciML dropped support. Activates errors on deprecation. Not sure what to do with the tests in the legacy file, since these will now error.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 2.56%. Comparing base (733a26a) to head (6a04e0f).

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #60       +/-   ##
===========================================
- Coverage   100.00%   2.56%   -97.44%     
===========================================
  Files            6       5        -1     
  Lines           65      39       -26     
===========================================
- Hits            65       1       -64     
- Misses           0      38       +38     

Flag	Coverage Δ
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[Vaibhavdixit02]

I think we want to allow the deprecation warnings to pass here. Otherwise, we'd need downstream breaking releases which isn't ideal

[gdalle]

There are packages outside of SciML which use ADTypes and want to preserve compatibility with 1.6. Removing it is not okay.

Why do you want to activate error on deprecation? We deprecated things on purpose in v1 to allow for a smooth transition

@ChrisRackauckas this is a disruptive PR, please don't merge until I'm okay with it