Canvas: Promote Button to beta

[adela-almasan]

This PR

• promotes the Canvas Button to beta state
• removes check for disableSanitizeHtml feature flag
• removes the disable check for button - good to have in the future, but maybe in a better form

Fixes #76498

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[drew08t]

LGTM

[nordby]

WHT! net::ERR_CERT_AUTHORITY_INVALID Urgently add the --unsecured flag