fix(graphql): use date scalar type for timestamps

[ajohn25]

Description

This converts a few String timestamps scattered in the codebase to the custom Date GraphQL scalar type

Motivation and Context

Noticed bug with displaying template campaign creation date as shown in the screenshot here #224 The other timestamps affected by this PR aren't displayed to users

The Date scalar ensures the timestamps are returned as ISO strings, which is how they're generally worked with throughout the codebase. The serialization code for the type (from node_modules) :

serialize: function (value) {
  assertErr(value instanceof Date, TypeError, ...)
  return value.toJSON()   // -> proper ISO 8601 string, e.g. "2026-07-30T12:34:56.789Z"
}

How Has This Been Tested?

This has been tested locally

Screenshots (if appropriate):

Documentation Changes

Checklist:

• My change requires a change to the documentation.
• I have included updates for the documentation accordingly.