Remove pre-WP5.2 PHP polyfill code, including test code

[felixarntz]

🚨 Note: do not merge until Release 1.88.0 🚨

Together with bumping the minimum WordPress version requirement to 5.2 (see #5874), we should remove any polyfill code that is only in Site Kit for WordPress versions lower than 5.2.

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Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• Any polyfill code that is only present in the plugin to cater for WordPress versions below 5.2 (i.e. any WordPress version between 4.7 and 5.1) should be removed.
• This should apply to the entire codebase (PHP and JS, production code and test/utility code).

Implementation Brief

This can be started, but it must not be merged until develop is open for the 1.88.0 release. When you open a pull request for this issue, please add the same annotation to the top of the PR to minimize the chance of an accidental early merge. :)

• Remove BC_Functions::rest_preload_api_request and BC_Functions::load_script_textdomain methods.
• Update the above methods' usage to use them directly from WordPress instead.
• Delete packages/wp-phpunit-polyfill directory and remove it as a dependency in composer.json.

Test Coverage

• Fix any failing tests.

QA Brief

• This PR doesn't include any visual/behavioural changes. This just changes how a few methods are sourced in the back-end.
• To ensure there is no regression, you can go through setting up Site Kit, setting up modules, and verifying everything loads and looks well in the Site Kit Dashboard and Settings.
• Verify that there are no console errors or network request failures.

Changelog entry

• Remove now unnecessary polyfills for WordPress versions older than 5.2, according to new version requirement.

[techanvil]

IB ✅

[tofumatt]

Moving this to the next sprint and adding a note to the issue that this shouldn't be merged until 1.88.0. 👍🏻

[wpdarren]

QA Update: ✅

Verified:

• Ensured there are no regressions when going through setting up Site Kit, setting up modules.
• Verified everything loads in the Site Kit Dashboard and Settings.
• There are no console errors or network request failures.