contrib/tzdata: correct DST in Fiji

Direct commit to stable/12.

Unfortunately, there is still no clear consensus on the tz mailing list
about some of the changes introduced by tzdata 2021b and later releases.
Pending consensus, only merge the recently announced DST transition date
for Fiji and corrections to commentary from tzdata 2021d. This corrects
future timestamps in Fiji.

Approved by: re (gjb)

(cherry picked from commit 74a0f31dbbd0b32689d63d8c965c611bdfc4da2f)