gate: specify close as noexcept
gate: specify close as noexcept

Although make_optional is not specified as noexcept,
it practicly is since it doesn't allocate any memory
and promise<> is nothrow constructible.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar