A few source files have the wrong copyright year

I noticed earlier today when working in brin_minmax_multi.c that the
copyright year was incorrect. That caused me to wonder if any other
source files have the incorrect year.

git grep -E "Portions Copyright \(c\) ([0-9]{4}-[0-9]{4}|[0-9]{4}),
PostgreSQL Global Development Group" | grep -Ev "2021"

Seems fairly good at finding them. 14 in total.

The attached fixes.

I'll push this in the New Zealand morning unless anyone comes up with
a reason why I shouldn't before then.

David