feat: fixed height for prerequisite course dropdown list

[ihor-romaniuk]

Backport: #1154 (original PR to master)
Related PR: #1555 (backport to redwood branch)

Description

made fixed height for the prerequisite course dropdown list.
Note: same height is set for the course language dropdown list.

Steps to reproduce

1. add waffle flag contentstore.new_studio_mfe.use_new_grading_page (in /admin/waffle/flag);
2. go to studio -> Settings -> Schedule & details;
3. scroll page till the end and click on Prerequisite course drop-down

BEFORE

Drop-down list goes beyond footer

AFTER

Drop-down list has the same height as Course language

Course language example

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Thanks for the pull request, @ihor-romaniuk!

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