Rename the "point is strictly above/below point" comparison operators.

Rename the "point is strictly above/below point" comparison operators.

Historically these were called >^ and <^, but that is inconsistent
with the similar box, polygon, and circle operators, which are named
|>> and <<| respectively.  Worse, the >^ and <^ names are used for
*not* strict above/below tests for the box type.

Hence, invent new operators following the more common naming.  The
old operators remain available for now, and are still accepted by
the relevant index opclasses too.  But there's a deprecation notice,
so maybe we can get rid of them someday.

Emre Hasegeli, reviewed by Pavel Borisov

Discussion: https://postgr.es/m/24348.1587444160@sss.pgh.pa.us