stack/slapos.cfg: upgrade slapos.libnetworkcache to 0.21 to support bootstrap with python3.9

See nexedi/slapos!1207 (comment 174788)

stack/slapos.cfg

@@ -176,7 +176,7 @@ slapos.cookbook = 1.0.167
 slapos.core = 1.6.2
 slapos.extension.strip = 0.4
 slapos.extension.shared = 1.0
-slapos.libnetworkcache = 0.20
+slapos.libnetworkcache = 0.21
 slapos.rebootstrap = 4.5
 slapos.recipe.build = 0.46
 slapos.recipe.cmmi = 0.16