Themes: Block theme activation if it requires a higher version of PHP or WordPress.

Introduce `validate_theme_requirements()` for validating a theme's WordPress and PHP version requirements.

Follow-up to [44978] and [45546] for plugins.

Props afragen, audrasjb, SergeyBiryukov.
See #43992.
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git-svn-id: https://core.svn.wordpress.org/trunk@47348 1a063a9b-81f0-0310-95a4-ce76da25c4cd