Comments: Rename the `$avoid_die` parameter of `wp_allow_comment()` and...

Comments: Rename the `$avoid_die` parameter of `wp_allow_comment()` and `wp_new_comment()` to `$wp_error`.

This makes the function signatures more consistent with `wp_update_comment()` and `wp_set_comment_status()`.

`wp_check_comment_flood()` is left as the only function with the `$avoid_die` parameter for now, as it does not return a `WP_Error` object.

Follow-up to [48154], [48207].

See #39732.
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