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 
Reviewed-by: Tom Lane 
Discussion: https://postgr.es/m/5D7E0F5E-E620-4D54-8788-66D421AC76F0@yesql.se
Backpatch-through: v12