databases/pg_tileserv: PIN to GO 1.20

This port deos not build at the moment unlike the other commits which I
have tested. For now comitting without tests as the other ports works
well with same changes.

There are also reports in portsfallout.

PR:		277776
Approved by:	portmgr (blanket)

databases/pg_tileserv/Makefile

@@ -11,7 +11,7 @@ WWW=		https://github.com/crunchydata/pg_tileserv/issues
 LICENSE=	APACHE20
 LICENSE_FILE=	${WRKSRC}/LICENSE.md
 
-USES=		go:modules
+USES=		go:1.20,modules
 USE_GITHUB=	nodefault
 GH_TUPLE=	golang:sys:v0.6.0:sys
 GO_MODULE=	github.com/crunchydata/pg_tileserv