doc: Specify when ssl_prefer_server_ciphers was added

 doc: Specify when ssl_prefer_server_ciphers was added
 
 The ssl_prefer_server_ciphers setting is quite important from a
 security point of view, so simply stating that older versions
 doesn't have it isn't very helpful.  This adds the version when
 the GUC was added to help readers.
 
 Backpatch to all supported versions since this setting has been
 around since 9.4.
 
 Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
 Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
 Discussion: https://postgr.es/m/5D7E0F5E-E620-4D54-8788-66D421AC76F0@yesql.se
 Backpatch-through: v12