pgsql: Sync our copy of the timezone library with IANA release tzcode20

Sync our copy of the timezone library with IANA release tzcode2020d.

There's no functional change at all here, but I'm curious to see
whether this change successfully shuts up Coverity's warning about
a useless strcmp(), which appeared with the previous update.

Discussion: http://mm.icann.org/pipermail/tz/2020-October/029370.html

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/34285083b368539ef9ad233cbe799209e304f286

Modified Files
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src/timezone/README | 2 +-
src/timezone/zic.c | 14 ++++++++++++--
2 files changed, 13 insertions(+), 3 deletions(-)