[Debian-rtc-admin] Upgrade prosody and prosody-modules to buster-backports versions on vogler

[Martin]

Hi,

I'll ask DSA to upgrade prosody and prosody-modules on vogler to
the buster-backports versions instead of the stable versions, if
nobody objects. Reason:

Some of my contacts get error messages, when I go online "in the
wrong moment". This seems to be solved in mod_smack
(prosody-modules) since 2020-05.  

Also, there are many bug fixes and improvements from prosody
0.11.2 to 0.11.7 and in the 1.5 years of development in the
prosody-modules.

Cheers