add support for Ubuntu Hirsute and Impish

The current `globals.pp` does not have the necessary default versions for Ubuntu 21.04 (Hirsute Hippo) and 21.10 (Impish Indri). Both of these versions have version 13 available, as can be seen below:

- Hisute => https://packages.ubuntu.com/hirsute/postgresql
- Impish => https://packages.ubuntu.com/impish/postgresql

Author: nicholascioli

manifests/globals.pp

@@ -205,6 +205,7 @@
         /^(17.04|17.10)$/ => '9.6',
         /^(18.04)$/ => '10',
         /^(20.04)$/ => '12',
+        /^(21.04|21.10)$/ => '13',
         default => undef,
       },
       default => undef,