Normative: Limit time zone offset precision to minutes

[gibson042]

This aligns with https://www.ietf.org/archive/id/draft-ietf-sedate-datetime-extended-08.html#section-4 , which allows expression of UTC offsets per RFC 3339 time-numoffset: https://www.rfc-editor.org/rfc/rfc3339#section-5.6

time-minute     = 2DIGIT  ; 00-59
…
time-numoffset  = ("+" / "-") time-hour ":" time-minute

Prompted by discussion in TG2 about https://github.com/tc39/ecma402/pull/788 .

[gibson042]

I don't know what "and id is valid as the first argument to GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds" means since those operations do not have any assertions or return completion records, which is all I could imagine this would mean.

GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds do each have a simple _timeZoneIdentifier_ is *"UTC"* assertion in the ECMA-262 default implementation, and this text can be simplified once Temporal GetAvailableNamedTimeZoneIdentifier makes it in.

-In the latter case, the result from AvailableNamedTimeZoneIdentifiers() contains a Time Zone Identifier Record _r_ such that _r_.[[Identifier]] is _id_, and _id_ is valid as the first argument to GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds.
+In the latter case, GetAvailableNamedTimeZoneIdentifier(_id_) returns a Time Zone Identifier Record.