Fix GH-11281: DateTimeZone::getName() does not include seconds in offset

If the seconds portion is non-zero, include the seconds in the output.

Closes GH-11282.

Adjust existing tests for fixed bugs

Upgrade timelib to 2021.06

Fixes among others:
. Bug #79580 (date_create_from_format misses leap year).
. Bug #80974 (Wrong diff between 2 dates in different timezones).
. Bug #8

Upgrade timelib to 2021.06

Fixes among others:
. Bug #79580 (date_create_from_format misses leap year).
. Bug #80974 (Wrong diff between 2 dates in different timezones).
. Bug #81097 (DateTimeZone silently falls back to UTC when providing an offset with seconds).
. Bug #81273 (Date interval calculation not correct).

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