CSS: Remove unnecessary opacity for Firefox placeholders.

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/63030

This PR removes unnecessary opacity: 1 rules for ::-moz-placeholder selectors in Core and bundled themes. Firefox changed its placeholder implementation from opacity-based to using color-mix, making these overrides redundant while maintaining color declarations for accessibility.

[github-actions]

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[github-actions]

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[sabernhardt]

Please leave the theme files unchanged in this pull request. WordPress (administration) limits its browser support to the latest versions of Firefox, but the themes could retain more support for older browsers.

Also, I think the :-ms-input-placeholder rulesets can be removed from the color-picker, forms and media-views stylesheets at this time. I'll mention that in the ticket for more discussion.

[himanshupathak95]

Please leave the theme files unchanged in this pull request. WordPress (administration) limits its browser support to the latest versions of Firefox, but the themes could retain more support for older browsers.

Also, I think the :-ms-input-placeholder rulesets can be removed from the color-picker, forms and media-views stylesheets at this time. I'll mention that in the ticket for more discussion.

Thanks for the review @sabernhardt, I have removed the :-ms-input-placeholder rulesets at this time, but I'll revert them if the discussion says otherwise.