metadata.json: increase puppetlabs/stdlib lower bound to 8.4.0

Commit 7dd0828 (#445) introduced
stdlib::deferrable_epp() which was introduced in stdlib 8.4.0 in
<puppetlabs/puppetlabs-stdlib@c1127f2>.

Author: kenyon

metadata.json

@@ -18,7 +18,7 @@
     },
     {
       "name": "puppetlabs/stdlib",
-      "version_requirement": ">= 4.25.0 < 10.0.0"
+      "version_requirement": ">= 8.4.0 < 10.0.0"
     },
     {
       "name": "puppet/archive",