Rename SLRU elements in view pg_stat_slru

Rename SLRU elements in view pg_stat_slru

The new names are intended to match those in an upcoming patch that adds
a few GUCs to configure the SLRU buffer sizes.

Backwards compatibility concern: this changes the accepted names for
function pg_stat_slru_rest().  Since this function recognizes "any other
string" as a request to reset the entry for "other", this means that
calling it with the old names would silently reset "other" instead of
doing nothing or throwing an error.

Reviewed-by: Andrey M. Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/202402261616.dlriae7b6emv@alvherre.pgsql