feat!: canonicalise union of indexed

[agoose77]

This PR:

• Deprecates the merge argument to UnionArray.simplified — since the previous introduction of canonicalisation checks, this argument can no longer be meaningfully False in contexts where layouts are mergeable.
• Add a new rule for the simplification of IndexedArrays inside unions.

Closes #2192

[codecov]

Codecov Report

Merging #2527 (85a1fb0) into main (f928ba2) will decrease coverage by 0.04%.
The diff coverage is 90.90%.

❗ Current head 85a1fb0 differs from pull request most recent head 92e1e36. Consider uploading reports for the commit 92e1e36 to get more accurate results

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/content.py	76.03% <ø> (ø)
src/awkward/contents/indexedoptionarray.py	88.59% <ø> (ø)
src/awkward/contents/unionarray.py	84.76% <90.90%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes

[jpivarski]

I know we had this conversation, but I'll reiterate that I agree that IndexedArray inside a UnionArray can get absorbed into the UnionArray's index, and therefore should be. Unless it's categorical, which is a user-visible type.

I added the UnionArray.simplified merge-argument deprecation to the wiki. (And maybe was too vocal about it being more like a bug-fix than a deprecation, but I don't think it matters what we call it.)

You can merge when ready!