Adjust for API changes in AdvancedHMC.jl

[Micki-D]

This checks if rand_momentum() from AdvancedHMC.jl is defined and if true uses this over the deprecated rand() method in older versions (<0.7) of AdvancedHMC.jl.

[oschulz]

Test failures were due to outdated dependency bounds of MGVI.jl - that's fixed now, let's see if tests pass now.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 50.08%. Comparing base (c781586) to head (1d959ea).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/ahmc_impl/ahmc_sampler_impl.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #470      +/-   ##
==========================================
- Coverage   50.08%   50.08%   -0.01%     
==========================================
  Files         122      122              
  Lines        5872     5874       +2     
==========================================
+ Hits         2941     2942       +1     
- Misses       2931     2932       +1     

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