PBCKP-816 Store the GPG key in the recommended way

Author: postgres-dev

doc/pgprobackup.xml

@@ -466,8 +466,8 @@ doc/src/sgml/pgprobackup.sgml
       Add the <application>pg_probackup</application> repository GPG key
       <programlisting>
 sudo apt install gpg wget
-wget -O - https://repo.postgrespro.ru/pg_probackup/keys/GPG-KEY-PG_PROBACKUP | \
-sudo apt-key add -
+wget -qO - https://repo.postgrespro.ru/pg_probackup/keys/GPG-KEY-PG_PROBACKUP | \
+sudo tee /etc/apt/trusted.gpg.d/pg_probackup.asc
 </programlisting>
     </listitem>
     <listitem>