Fix disabled menu item button styles

[talldan]

Description

This is a small fix for a visual regression introduced in #16103.

In that PR an opacity of 0.3 that was applied to any disabled button was removed. This caused disabled menu items to no longer appear disabled. This screenshot shows a menu with some enabled and some disabled menu items when that opacity is no longer present:

This PR reintroduces that opacity style, but only for menu items:

How has this been tested?

1. Add a table block to a post
2. Without selecting a cell, click on the edit toolbar button on the edit toolbar

Expected (in this branch)

Buttons in the dropdown menu appear greyed out.

Actual (in master)

Buttons in the dropdown menu appear active, but cannot be clicked on or interacted with

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[mapk]

Thanks for catching this Dan! I was trying to think about where else this opacity change might occur and didn't even consider this one. Thanks for the fix! 👍

I tested this PR, and it works as expected. Let's get this in.

[tellthemachines]

The code looks good! Left a comment but it's more of a question.
Fwiw this is what it looks like in high contrast mode on Windows:

(not sure if there's a better way of indicating disabled status for low vision users though)

[youknowriad]

Was this fixed in a more global way @aduth? Should we revert that change in that case as it becomes unnecessary styles?

[aduth]

Yes, this treats a symptom of the root issue which is that, as of #16103 (#16103 (comment)), we have no disabled stying for buttons.

I think the changes there (at least this one line) should be reverted, or find another way which treats exceptions to default disabled styling.

[aduth]

See #16769