Replace column index usage in db column alter operations with column attnum.

[silentninja]

Fixes #993

Technical details

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[codecov-commenter]

Codecov Report

Merging #1132 (79a663d) into master (1a831c1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1132   +/-   ##
=======================================
  Coverage   93.37%   93.38%           
=======================================
  Files         112      112           
  Lines        4286     4291    +5     
=======================================
+ Hits         4002     4007    +5     
  Misses        284      284           

Flag	Coverage Δ
pytest-backend	93.38% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
db/columns/operations/alter.py	96.66% <100.00%> (+0.09%)	⬆️
db/columns/operations/infer_types.py	97.14% <100.00%> (+0.08%)	⬆️
mathesar/api/db/viewsets/columns.py	90.80% <100.00%> (ø)
mathesar/models.py	96.35% <100.00%> (ø)

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[dmos62]

Looks good. One minor change requested.