Rebrand `Remove Unused CSS` to `Reduce Unused CSS` to reflect what the feature does

[DahmaniAdame]

Is your feature request related to a problem? Please describe. Remove Unused CSS feature name doesn't reflect what the feature does and how it operates.

It leads to misinterpretation and a high expectation to have a 100% removal of all used CSS, which would result in page and elements breaking because of hidden elements, a common practice on WordPress.

Describe the solution you'd like Rebranding the feature to Reduce Unused CSS will fit the expectation and be paired with the target Lighthouse audit that the feature improves.

In Plugin > Quick Actions:

• Remove Used CSS Cache section is renamed to Reduce Unused CSS
• Clear used CSS button is renamed to Clear Optimizations

In Plugin > File optimization > Optimize CSS delivery:

• Remove Unused CSS button is renamed Reduce Unused CSS
• Description is updated with Reduces unused CSS instead of Remove unused CSS

Misc:

• Notices are mentioning Unused CSS optimizations instead of Used CSS cache
• Lighthouse stack packs: https://github.com/GoogleChrome/lighthouse-stack-packs/blob/master/packs/wp-rocket.js
• Docs are updated: https://docs.wp-rocket.me/article/1529-remove-unused-css
• Check for any messages visible in the console, logs, error messages, etc.

Describe alternatives you've considered N/A

Additional context N/A

[benorfaz]

@DahmaniAdame I've updated it with some requirements to not miss any place where we need to replace content, I may have missed some

[benorfaz]

Cancelling this change, and replaced by another issue with only minor copy changes https://github.com/wp-media/wp-rocket/issues/6768