Bump WP version requirement to 6.4+

[swissspidy]

Analytics numbers look good, so by the time we release our new version, only very few people will be on older versions.

Previously:

• Bump WP version requirement to 6.3+ #13515

Field guide: https://make.wordpress.org/core/2023/10/23/wordpress-6-4-field-guide/

Some of the changes around option autoloading are nice. New functions get_options(), wp_prime_option_caches(), and wp_set_option_autoload_values() allow an enhanced performance of retrieving options from the database. This would help unblock #13701.

Post meta revisions are also useful. In the Story_Post_Type class we can use the wp_post_revision_meta_keys filter to ensure POSTER_META_KEY and PUBLISHER_LOGO_META_KEY are part of revisions too. Might require some testing. Ditto for the Product_Meta class

Nothing else stands out at the moment.

Tasks include:

• Bump in readme.txt
• Bump in web-stories.php (twice)
• Update minimum_supported_wp_version in phpcs.xml.dist
• Run e2e tests against 6.4 and newer
• Run unit tests against 6.4 and newer
• Re-evaluate PHPUnit matrix and composer dependencies if necessary