Make WP_Automatic_Upgrader a proper object that gets instantiated. Renames nearly all of its methods.

Also renames wp_auto_updates_maybe_update() to wp_maybe_auto_update().

see #22704.

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Bundled Themes: pinking shears, fix a trailing period.

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Remove the old wp_auto_updates_maybe_update cron event. Schedule the new wp_maybe_auto_update event at 7 a.m. and 7 p.m. in the site's timezone.

see #27704.

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Inline documentation for hooks in wp-admin/customize.php.

Props andg, rzen.
Fixes #25444.

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Avoid a notice in is_main_network() when called in single site. see #25030.

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For Background updates, ensure that only one update process runs at the same time by using the options table as a lock.
This prevents multiple cron spawns and/or long-running updates from causing multiple update processes to spin up.
This also fixes a case where the upgrader might kick in for ( ! is_main_network() || ! is_main_site() ) in mulisite installs.
See #22704

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Inline documentation for hooks in wp-admin/options-discussion.php.

Props siobhyb, ericlewis.
Fixes #25453.

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Inline documentation for hooks in wp-admin/options-general.php.

Props siobhyb for the initial patch.
Fixes #25454.

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