core: Don't use "time" tag without "server-time"

Don't use the "time" IrcTagKey without "server-time" being enabled.
This ensures that if "server-time" isn't negotiated, e.g. due to
skipping the capability, message timestamps won't be adjusted.

According to the IRCv3 specifications, servers must not send the
"time" tag without "server-time" being acknowledged, however it's
probably better to handle the case where a server might send it
unprompted.

See https://ircv3.net/specs/extensions/server-time-3.2