Icon: Deprecate dashicon strings

[mirka]

It was suggested in #27461 (comment) that dashicons strings be deprecated from the Icon component, in favor of SVGs.

Note that this will also affect components that rely on Icon and accept arbitrary icon values, such as Button and DropdownMenu.

[mirka]

It has come to my attention that the Icon component is responsible for rendering block icons, where I would imagine dashicons to be widely used.

So this itself could be considered a blocker to deprecating.

[t-hamano]

Mentions #44205 as it may be relevant to this issue.