Handle AT_DropExpression in process_utility

[svenklemm]

PG13 adds a new ALTER TABLE subcommand for dropping the generated
property from a column.

postgres/postgres@f595117e24

[codecov]

Codecov Report

Merging #2796 (3b3bdb5) into master (6cc9871) will increase coverage by 0.21%.
The diff coverage is 92.10%.

@@            Coverage Diff             @@
##           master    #2796      +/-   ##
==========================================
+ Coverage   90.01%   90.23%   +0.21%     
==========================================
  Files         213      212       -1     
  Lines       34402    34620     +218     
==========================================
+ Hits        30968    31240     +272     
+ Misses       3434     3380      -54     

Impacted Files	Coverage Δ
src/bgw/job_stat.c	86.80% <0.00%> (ø)
src/catalog.h	100.00% <ø> (ø)
src/chunk.h	100.00% <ø> (ø)
src/compat.h	100.00% <ø> (ø)
src/compression_with_clause.c	87.06% <ø> (ø)
src/extension_utils.c	81.39% <ø> (ø)
src/import/planner.c	70.30% <ø> (+11.12%)	⬆️
src/indexing.c	96.15% <ø> (+1.28%)	⬆️
src/jsonb_utils.c	81.70% <ø> (-4.88%)	⬇️
src/loader/bgw_message_queue.c	87.09% <ø> (ø)
... and 126 more

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[gayyappan]

Looks good. Would be good to add a test case for this.

[pmwkaa]

+1 for test case, if possible