Remove extend from dropdown-menu-toggle

What does this MR do and why?

For #323021 (closed)

Delete an @extends from dropdown-menu-trigger. This is probably gonna break a bunch of stuff horribly.

My biggest concern is that dropdown-menu-toggle inherits a bunch of rules from dropdown-toggle just slightly further up in dropdowns.scss. That includes some padding, background-color, and border rules. dropdown-menu-trigger makes it into startup-general.css, because it is on the project page, but the other two rules there do not. So to actually test the impact meant also updating startupcss with the output from https://gitlab.com/gitlab-org/gitlab/-/jobs/1611389343

With that I was able to see broken dropdowns looked like this (this is the Sort dropdown on Projects list page):

So, to account for that I added dropdown-menu-trigger to the selectors list above, which I think is the only styles that should be required. After that the dropdown was back to its old self again

Screenshots or screen recordings

before/after for application.css https://gitlab.com/-/snippets/2178955

How to set up and validate locally

Apply this diff to remove the startupCSS rules:

1. Search for any of the dropdowns that only have the dropdown-menu-trigger class. Any gl-dropdowns (using toggle-class prop) or gl-buttons already add most of these styles. Though there may also be some rules that affect those buttons, but it is unlikely.

MR acceptance checklist

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