doc: Remove reference to trigger file regarding promotion

The wording changed here comes from 991bfe11d2, when the only way to
trigger a promotion was with a trigger file.  There are more options to
achieve this operation these days, like the SQL function pg_promote() or
the command `pg_ctl promote`, so it is confusing to assume that only a
trigger file is able to do the work.

Note also that promote_trigger_file has been removed as of cd4329d939
in 16~.

Author: Shinya Kato
Discussion: https://postgr.es/m/201b08ea29aa61f96162080e75be503c@oss.nttdata.com
Backpatch-through: 12

doc/src/sgml/high-availability.sgml

@@ -646,7 +646,7 @@ protocol to make nodes agree on a serializable transactional order.
     later disconnected, the standby goes back to step 1 and tries to
     restore the file from the archive again. This loop of retries from the
     archive, <filename>pg_wal</filename>, and via streaming replication goes on until the server
-    is stopped or failover is triggered by a trigger file.
+    is stopped or is promoted.
    </para>
 
    <para>