pgsql: doc: Remove reference to trigger file regarding promotion

doc: Remove reference to trigger file regarding promotion

The wording changed here comes from 991bfe11d28a, 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 cd4329d9393f
in 16~.

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

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fcf46f8fb76ca8086119b3f1510533332aefd604

Modified Files
--------------
doc/src/sgml/high-availability.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)