Backport SSL_CTX_set1_groups_list() from upstream PG18 (commit 3d1ef3a15c3)
to replace the legacy OBJ_sn2nid()/EC_KEY_new_by_curve_name() API in
initialize_ecdh(). This enables ssl_ecdh_curve to accept colon-separated
lists of TLS group names, including post-quantum hybrid groups such as
X25519MLKEM768.
Changes:
- Replace legacy ECDH API with SSL_CTX_set1_groups_list()
- Backport SSLerrmessageExt() helper for proper error messages
- Add errhint for invalid group name diagnostics
- Update GUC description to document colon-separated list support
- Update postgresql.conf.sample with explanatory comment
Resolves: RHEL-119229
Co-authored-by: Cursor <cursoragent@cursor.com>
