Support Julia 1.6 (again)

[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

This PR makes EnzymeCoreExt and ChainRulesExt unconditional dependencies for Julia <1.9, thus allowing support for older Julia versions.
We initially decided against it in #40 (comment), but as shown by #47 some users would like this feature. It is zero-cost for us, so why not?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (aa2429e) to head (fa3733a).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           64        64           
=========================================
  Hits            64        64           

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[gdalle]

@ChrisRackauckas @Vaibhavdixit02 I think this doesn't hurt, right?

[gdalle]

Bump @Vaibhavdixit02

[ChrisRackauckas]

If you all want to maintain it, go for it. We don't plan to put any effort into the old LTS since the new one is already out.

[gdalle]

I didnt have that in mind, so all of SciML is basically willing to drop 1.6?

[ChrisRackauckas]

Yes.