remove deprecated html tags from default valid tags

[spereverde]

According to this:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element#obsolete_and_deprecated_elements
the following html tags are deprecated:

• acronym
• big
• tt
  They are however still added in the default list of valid tags:
  eg. https://github.com/plone/plone.base/blob/main/src/plone/base/interfaces/controlpanel.py#L234

Do we want to keep these for some reason, or should I make a PR to remove them from the default?

[mauritsvanrees]

Probably fine to remove them indeed.

I would not do a migration: it seems safer to me when existing sites keep their current settings, and this only applies to new sites.