pgsql: Fix EXTRACT(ISOYEAR FROM timestamp) for years BC.

Fix EXTRACT(ISOYEAR FROM timestamp) for years BC.

The test cases added by commit 26ae3aa80 exposed an old oversight in
timestamp[tz]_part: they didn't correct the result of date2isoyear()
for BC years, so that we produced an off-by-one answer for such years.
Fix that, and back-patch to all supported branches.

Discussion: https://postgr.es/m/SG2PR06MB37762CAE45DB0F6CA7001EA9B6550@SG2PR06MB3776.apcprd06.prod.outlook.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/332584da9cfdbe91a651427a9fc3ed76dffaa529

Modified Files
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src/backend/utils/adt/timestamp.c         | 7 +++++++
src/test/regress/expected/timestamp.out   | 2 +-
src/test/regress/expected/timestamptz.out | 2 +-
3 files changed, 9 insertions(+), 2 deletions(-)