Modify schema to enforce UTC and timezones

[jayjacobs]

Currently the timestamp values in the record are all over the place (see AWG #135)

These are a list of changes the schema to enforce clean storage of time stamps:

• Modify the "timestamp" definition to reference UTC instead of GMT (Use UTC instead of GMT in JSON5 #289)
• Make the inclusion of timezones non-optional (alternatively coordinate with CVE Services to ensure their issue 1274 will handle whatever input is allowed by the schema.
• Remove "datestamp" definition field as it is not referenced or used anywhere in the schema. (the "timestamp" definition is referenced everywhere a date/time object is needed.

[ccoffin]

Discussed in 10/31/2024 QWG. Three changes suggested above by Jay.

[sei-vsarvepalli]

I would recommend if we require it we specify RFC 3339 in the documentation - being very specific about the format we want to support ISO8601 is broader - https://www.rfc-editor.org/rfc/rfc3339