WPDB Unit Tests: Fix a fatal error that occurs in the `test_strip_invalid_text()` test if the `mbstring` extension isn't activated in PHP.

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Document the `$user_id` parameter added to the 'admin_color_scheme_picker' hook in 3.8.1.

Props Ipstenu.
Fixes #30551.

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Remove return descriptions from the DocBlocks for `add_image_size()` and `set_post_thumbnail_size()` as they don't return anything.

Also document the `$_wp_additional_image_sizes` global, leveraged in `add_image_size()`.

Props stevegrunwell.
Fixes #30511.

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Upgrade to MediaElement 2.16.2
https://github.com/johndyer/mediaelement/compare/2.15.1...2.16.2

Fixes #30281.

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Remove an invalid parameter from the DocBlock for the `gallery_style` hook.

Props keesiemeijer.
Fixes #30507.

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Ensure uniqueness when returning page lists in `get_page_children()`. Fixes failing unit tests.

Props boonebgorges.
Reverts [30246].
Fixes #14477.

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IETF language changes in MediaElement:
https://github.com/johndyer/mediaelement/pull/1270/

Reverts [28801].
Fixes #29182.

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