golang: Switch default to Go1.21

/reviewed-by @kirr and @jerome
/reviewed-on nexedi/slapos!1494

component/golang/buildout.cfg

@@ -202,7 +202,7 @@ bin	= ${gowork.dir:bin}
 depends = ${gowork.goinstall:recipe}
 
 # go version used for the workspace (possible to override in applications)
-golang  = ${golang1.20:location}
+golang  = ${golang1.21:location}
 
 # no special build flags by default
 buildflags =