Pass `$post` to `post_password_required()` in `get_the_excerpt()`

[sebastian.pisula]

Two fix in get_the_excerpt function:

1. in docs param $post can be int|WP_Post and null
2. post_password_required function should have $post param

[swissspidy]

int|WP_Post|null is used like two or three times throughout the code base, whereas int|WP_Post is used everywhere else. null is the default value and IMHO already covered by int|WP_Post.

The post_password_required() part is a bad oversight though.

[swissspidy]

In 36329:

Posts: Correctly pass $post to post_password_required() in get_the_excerpt().

Corrects the relevant test.

Props sebastian.pisula for initial patch.
Fixes #35486.