pgsql: Add X25519 to the default set of curves

Add X25519 to the default set of curves

Since many clients default to the X25519 curve in the TLS handshake,
the fact that the server by defualt doesn't support it cause an extra
roundtrip for each TLS connection. By adding multiple curves, which
is supported since 3d1ef3a15c3eb68da, we can reduce the risk of extra
roundtrips.

Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Co-authored-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/20240616234612.6cslu7nqexquvwj7@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/daa02c6bd9262adeb44f4a9ed9d94fa6259afd94

Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
src/backend/utils/misc/guc_tables.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
src/test/ssl/t/SSL/Server.pm | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)