Fix openapi docs of UTCTime and UTCTimeMillis

[pcapriotti]

Fix openapi docs inconsistency with datetime fields. We have two kinds of datetime fields: with and without a millisecond component. They were both documented as having the millisecond component.

https://wearezeta.atlassian.net/browse/WPB-6529

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines
• [x] Add note to "API changes" page