Upgrade/Install: Use `is_php_version_compatible()` and...

Upgrade/Install: Use `is_php_version_compatible()` and `is_wp_version_compatible()` in plugin and theme requirement checks.

Follow-up to [48390].

Props afragen.
See #9757.
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git-svn-id: https://core.svn.wordpress.org/trunk@48217 1a063a9b-81f0-0310-95a4-ce76da25c4cd