sp-menu should be deprecated from popover menu classes

[adixon-adobe]

Expected Behaviour

Our APIs have minimal DOM structure, and limits invalid usage when possible.

Actual Behaviour

As illustrated in #1189, the underlying sp-menu can lead to invalid DOM states, and is ultimately redundant with the sp-picker, sp-split-button & sp-action-menu elements when used in that context.

Proposal

Deprecate the sp-menu element in these contexts, and use console.warn to indicate this usage pattern will be removed in a future version.

[Westbrook]

@adixon-adobe was there more work that you wanted to do to this issue post #1237?

[adixon-adobe]

No - the next round will be tackling #1189 so I'll close this!