nixos: fix services.foundationdb.traceFormat for older server versions

This was a testing oversight that came from #61009 -- I forgot to test
the new traceFormat option with older server versions while I was
working on FDB 6.1.

Since trace_format is only available in 6.1+, emitting it
unconditionally caused older versions of the database fail to start,
reporting an error. We simply gate it behind a version check instead,
and assert the format is always XML on older versions. This avoids the
case where the user has an old version, changes traceFormat willingly,
and then is confused by why it didn't work.

As reported by @TimothyKlim in the comments on commit
c55b9236. See

    https://github.com/NixOS/nixpkgs/commit/c55b9236f0f208a8689e400ddbea52858c923372#r33566132



Signed-off-by: Austin Seipp <aseipp@pobox.com>