avoid hard-coding height in language dropdown menu (#25986)

avoid hard-coding height in language dropdown menu (#25986)

This commit removes the hard-coded height of 500px, using that as a
max-height instead. The height of items in the dropdown menu, assuming a
default font size of 16px, is 36px, so the old CSS would cause overly
large dropdown menus in instances where less than 14 languages are
offered.

Refs: https://codeberg.org/forgejo/forgejo/pulls/1000

Co-authored-by: rome-user <rome-user@noreply.codeberg.org>
Co-authored-by: Giteabot <teabot@gitea.io>