label and assorted a11y fixes to timestamp field

[gwyneplaine]

• timestamp field now wrapped in fieldset and given a legend element, rathern than a div and label respectively
• adjusted date picker clear value button to have SR friendly clear icon
• added visually hidden label for time input for better SR experience.

[changeset-bot]

🦋 Changeset detected

Latest commit: f36652e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@keystone-ui/fields	Patch
@keystone-next/fields	Patch

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