Tighten [style*="border-color"] selector to avoid matching URLs in inline styles

[benniledl]

This change fixes an issue where the selector [style*="border-color"] was too broad and matched unintended cases.

On the WordCamp Europe site, an inline style contained a background image URL with border-color in the filename. This caused the rule to incorrectly apply borders, resulting in random borders appearing across the site.

The selector is now tightened to [style*="border-color:"] to ensure it only targets actual CSS declarations, not substrings inside URLs.

Related #75546
@aaronrobertshaw
AI Note: PR Title and Description written by AI

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[aaronrobertshaw]

@benniledl Thanks for the PR it would be great if you could update the PR description to follow the Gutenberg project's template as it will provide a better path to review, provide required context and in this particular case help evaluate alternative solutions e.g. #77476

[talldan]

I think this fix might still match css vars, so it'd be good to avoid that, as it's the main issue reported in #70211

[aaronrobertshaw]

As the approach in this PR doesn't solve the primary issue being reported. I'll close this in favour of alternative approaches already noted in this PR discussion.