databases/pgbackrest: update to 2.51

Switch to meson build system.
Rename option SSH to SFTP.

The origin of patch is
https://github.com/pgbackrest/pgbackrest/commit/7b95fd3bd29346e7325aa161bfee45efd71e22eb
It is upstreamed and can remove it at update to 2.52.

Changelog: https://pgbackrest.org/release.html#2.51

PR: 276559
Approved by: diizzy (mentor)
Differential Revision: https://reviews.freebsd.org/D44791