Stop using Base.get_extension.

[GunnarFarneback]

According to JuliaLang/julia#59593 it's a bad idea to use Base.get_extension and this alternative arguably looks nicer anyway.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.31%. Comparing base (5ed108b) to head (f1a668d).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/CUDAExt.jl	0.00%	4 Missing ⚠️
src/highlevel.jl	0.00%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   88.37%   86.31%   -2.06%     
==========================================
  Files           4        4              
  Lines         387      402      +15     
==========================================
+ Hits          342      347       +5     
- Misses         45       55      +10     

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[GunnarFarneback]

Julia nightly fails due to BFloat16s only working on nightly from 0.6.0 while some other compat keeps it down to 0.5.1. I think it's too early to worry about nightly considering that 1.12 was just released, and in any case that problem is unrelated to this PR.

[jw3126]

Thanks LGTM.