Social Icons: Disable edit as HTML support

[shimotmk]

What?

This PR disables HTML editing support for list blocks.
A similar PRs: #11785, #39318, #49426 #55656.

Why?

Editing these wrapper blocks in HTML directly is fragile and prone to breaking the users' content. HTML editing should be left to leaf-blocks.

#11785 (comment)

Testing Instructions

1. Open a Post or Page.
2. Insert a Social Icons block.
3. Confirm "Edit as HTML" action isn't available in block Options.

Screenshots or screencast

Before	After

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Co-authored-by: shimotmk <shimotomoki@git.wordpress.org>
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[t-hamano]

LGTM! Given the rules in other blocks, this PR makes sense.

[shimotmk]

Thanks for the review!