Split update and downgrade version

[mkindahl]

During an update, it is not possible to run the downgrade scripts until
the release has been tagged, but the update scripts can be run. This
means that we need to split the previous version into two different
fields: one for running the update tests and one for running the
downgrade tests.

[codecov]

Codecov Report

Merging #3381 (24c1401) into master (001f5d9) will decrease coverage by 1.56%.
The diff coverage is 88.45%.

@@            Coverage Diff             @@
##           master    #3381      +/-   ##
==========================================
- Coverage   92.17%   90.60%   -1.57%     
==========================================
  Files         110      212     +102     
  Lines       14177    35628   +21451     
==========================================
+ Hits        13067    32281   +19214     
- Misses       1110     3347    +2237     

Impacted Files	Coverage Δ
src/bgw/launcher_interface.c	72.22% <ø> (+1.63%)	⬆️
src/catalog.h	100.00% <ø> (ø)
src/chunk.c	95.90% <ø> (+1.37%)	⬆️
src/chunk.h	100.00% <ø> (ø)
src/chunk_adaptive.c	92.57% <ø> (+2.78%)	⬆️
src/chunk_append/chunk_append.c	97.41% <ø> (+0.90%)	⬆️
src/chunk_append/exec.c	95.84% <ø> (+1.66%)	⬆️
src/chunk_append/explain.c	82.81% <ø> (ø)
src/chunk_append/planner.c	93.38% <ø> (-2.27%)	⬇️
src/chunk_append/transform.c	96.66% <ø> (-3.34%)	⬇️
... and 280 more

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