Use UTC instead of GMT in JSON5

[eslerm]

The description on timestamp states:

Date/time format based on RFC3339 and ISO ISO8601, with an optional timezone in the format 'yyyy-MM-ddTHH:mm:ssZZZZ'. If timezone offset is not given, GMT (0000) is assumed.

cve-schema/schema/v5.0/CVE_JSON_5.0_schema.json

Line 79 in 0e0a4ac

"description": "Date/time format based on RFC3339 and ISO ISO8601, with an optional timezone in the format 'yyyy-MM-ddTHH:mm:ssZZZZ'. If timezone offset is not given, GMT (0000) is assumed.",

Both RFC3339 and ISO ISO8601 have deprecated GMT in favor of UTC.

GMT is not strictly synonymous with UTC. GMT can be used to refer to UTC or UT1. GMT is based on astronomical events and UTC is based on atomic clocks.