Cirrus-CI: reduce VM memory to 8G

Cirrus-CI now provides a task memory use graph, and it is clear we do
not need to provision the VM with 24GB.

Sponsored by:	The FreeBSD Foundation

(cherry picked from commit bbf7027055)

.cirrus.yml

@@ -3,7 +3,7 @@
 freebsd_instance:
   image: freebsd-12-2-release-amd64
   cpu: 8
-  memory: 24G
+  memory: 8G
 
 env:
   CIRRUS_CLONE_DEPTH: 1