Remove unnecessary opacity override for Firefox placeholders

[himanshupathak95]

What?

Closes #69311

This PR removes the obsolete opacity: 1 CSS property from ::-moz-placeholder selectors across, as Firefox has updated its placeholder implementation.

Why?

Since Firefox now implements placeholder transparency via the color property instead of opacity, our opacity: 1 override is no longer necessary and can be safely removed.

The visual appearance remains unchanged in current Firefox versions as setting just the color property correctly overrides Firefox's native transparency.

Testing Instructions

• Update to the latest version of Firefox
• In Firefox, navigate to about:config and set devtools.inspector.showAllAnonymousContent to true
• Edit a post
• Switch the editor to 'Code editor'
• Make sure the textarea is empty to see the placeholder text
• Inspect the placeholder text in Firefox DevTools
• Verify that the placeholder text appears correctly without the opacity: 1 property

Screenshots

[github-actions]

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Co-authored-by: afercia <[email protected]>

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[himanshupathak95]

For the placeholder in the TextareaControl -

Screen.Recording.2025-02-25.at.23.20.26.mov

As can be seen, removing opacity doesn't make any change. So it can be safely removed. Screenshot after the removal (current PR) -

[afercia]

LGTM