btree_gist: Merge the last two versions into version 1.8

btree_gist: Merge the last two versions into version 1.8

During the development cycle of v18, btree_gist has been bumped once to
1.8 for the addition of translate_cmptype support functions (originally
7406ab623fee, renamed in 32edf732e8dc).  1.9 has added sortsupport
functions (e4309f73f698).

There is no need for two version bumps in a module for a single major
release of PostgreSQL.  This commit unifies both upgrades to a single
SQL script, downgrading btree_gist to 1.8.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/13c61807-f702-4afe-9a8d-795e2fd40923@illuminatedcomputing.com
Backpatch-through: 18