What problem does this address?

The FontSizePicker component currently has an inherent margin-bottom, making it hard to reuse in different contexts, since it requires a style override of an internal element. This kind of internal override is something we want to discourage.

Any outer margins, if desired, should be added by the consumer.

What is your proposed solution?

Deprecate the bottom margin (similar to #37160), following the guidelines stipulated in the devdocs.

Prerequisites:

• Part 1 of BaseControl: Deprecate bottom margin #38730
• Add flag on ToggleGroupControl FontSizePicker: Add flag to remove bottom margin #43062
• Add flag on RangeControl FontSizePicker: Add flag to remove bottom margin #43062