[Feature] Eliminate W3 Validation Error for <div> is not allowed child element of <label>

[jessecowens]

Is your feature request related to a problem? Please describe.
The W3 HTML5 validator at https://validator.w3.org/nu/ throws an error for the element ...
<div id="main-menu-hamburger" class="hamburger hamburger--collapse ">
...that a Div is not a valid child element of a label.

Describe the solution you'd like
Convert the hamburger menu toggle to a <span>, which is a valid child element of a label.

Additional context
This issue was reported on the WordPress.org Forums.