Increase max-height for merge request changed files dropdown

Note in this screenshot that only four items are visible at a time in a list of 1000:

In app/assets/stylesheets/framework/dropdowns.scss the default max-height for dropdowns is too low.

As mentioned in #39056 (closed), this is less than ideal for reviewing large merge requests. For an example of an MR where the small height is problematic, see !12841 (closed).

Proposal

Update the max-height for this dropdown's total height (including the search bar and padding) to 400px. This will allow for 7 rows to be shown at a time, while keeping scanability and small screens in mind.