test(pillar/debian): disable IdentLookups for Debian-11 [skip ci]

Similar situation to 0461052.

http://www.proftpd.org/docs/modules/mod_ident.html

`mod_ident` not available after installation:

```
root@dd4d32004e3c:~# proftpd -l
Compiled-in modules:
  mod_core.c
  mod_xfer.c
  mod_rlimit.c
  mod_auth_unix.c
  mod_auth_file.c
  mod_auth.c
  mod_ls.c
  mod_log.c
  mod_site.c
  mod_delay.c
  mod_facts.c
  mod_dso.c
  mod_auth_pam.c
  mod_cap.c
  mod_ctrls.c
  mod_lang.c
```

.yamllint

@@ -18,6 +18,7 @@ ignore: |
   test/**/states/**/*.sls
   .kitchen/
   kitchen.vagrant.yml
+  test/salt/pillar/debian.sls
   test/salt/pillar/redhat.sls
 
 yaml-files:

test/salt/pillar/debian.sls

@@ -1,5 +1,5 @@
 ---
 proftpd:
-  IdentLookups: '"off"'
+  IdentLookups: {{ '""' if grains.get('osfinger', '') in ['Debian-11'] else '"off"' }}
   User: nobody
   Group: nogroup