fix #3794: api types: set backup time lower limit to 1

fix #3794: api types: set backup time lower limit to 1

Some users want to import historical backups but they run into the
original lower backuo-time limit one can pass. That original limit
was derived from the initial PBS development start in 2019, it was
assumed that no older backup can exist with PBS before it existing,
but imports of older backups is a legitimate thing.

I pondered using 683071200 (1991-08-25), aka the first time Linux was
publicly announced by Linus Torvalds as new limit but at the end I
did not wanted to risk that and backup software is IMO to serious for
such easter eggs, so I went for 1, to differ between the bogus 0 some
tools fallback too if there's something off with time.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>