The zlib documentation mentions the values supported for the compression
strategy, but this code has been using a hardcoded value of 0 rather
than Z_DEFAULT_STRATEGY.  This commit adjusts the code to use
Z_DEFAULT_STRATEGY.

Backpatch down to where this code has been added to ease the backport of
any future patch touching this area.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/1400032.1662217889@sss.pgh.pa.us
Backpatch-through: 10