Move no-implicit-explicit-label check into label-content-name-mismatch rule

[WilcoFiers]

Part of #4066

The no-implicit-explicit-label check from aria-input/toggle-field-name will be moved to the label-content-name-mismatch experimental rule, and we'll downscale the impact from serious to moderate.

Update

After taking a closer look I no longer think the above was right. no-implicit-explicit-label never fails. It says that code like the following needs review:

<label>
  first name
  <div role="textbox" aria-label="name"></div>
</label>

This actually seems like a serious issue to me, if not critical. I think leaving this at moderate is incorrect. So instead of moving this check to an experimental rule, bumping the impact from moderate to serious is appropriate. User impact will be minimal as needs-review issues aren't counted in impact numbers.

[padmavemulapati]

Validated using latest axe-core develop branch code base,
for the code:

<label>
  first name
  <div role="textbox" aria-label="name"></div>
</label>

seeing the issue in incomeplete and serious