`_add_template_loader_filters` uses `add_filter` instead of `add_action`

[swissspidy]

_add_template_loader_filters uses add_filter to hook into pre_get_posts, which is an action hook and not a filter. Therefore, add_action should be used.

Technically, both are identical under the hood, but especially for people new to the code base this could cause confusion.

​https://github.com/WordPress/wordpress-develop/blob/f404ff34c0ce1b9fab35635f6b92f299a1442c7f/src/wp-includes/block-template.php#L14-L32

[davidbaumwald]

In 55335:

Themes: Use add_action for pre_get_posts hook in _add_template_loader_filters().

Although add_action and add_filter are internally the same, pre_get_posts is an action hook. This change updates the hook type to match what developers would expect when evaluating the code.

Follow-up to [52316].

Props swissspidy, riccardodicurti.
Fixes #57097.