feat: replace nplike with backend in to_buffers

[agoose77]

This follows up #1922 to remove any reference of nplike in the high level API, and renames nplikes to _nplikes

I also fixed the CUDA test generation, which I failed to fix in #1922

Note-to-self, write a git hook to run those.

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-feat-to-buffers-backend/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1942 (916e295) into main (16d482a) will decrease coverage by 0.00%.
The diff coverage is 94.18%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_connect/rdataframe/from_rdataframe.py	0.00% <0.00%> (ø)
src/awkward/_nplikes.py	68.33% <0.00%> (ø)
src/awkward/operations/ak_from_parquet.py	88.72% <0.00%> (ø)
src/awkward/operations/ak_transform.py	83.05% <ø> (ø)
src/awkward/_connect/numpy.py	66.19% <75.00%> (ø)
src/awkward/contents/numpyarray.py	89.62% <77.77%> (ø)
src/awkward/operations/ak_to_buffers.py	90.00% <80.00%> (-10.00%)	⬇️
src/awkward/operations/ak_to_categorical.py	92.15% <83.33%> (ø)
src/awkward/contents/unionarray.py	85.71% <85.71%> (-0.15%)	⬇️
src/awkward/behaviors/string.py	76.02% <88.88%> (ø)
... and 112 more

[jpivarski]

Good: nplikes is now hidden and users deal with backends everywhere instead. I've scanned over the differences twice and they look like straightforward changes.

I'd like to merge this right away, since it will be easier to merge into my big PR now, rather than later.