Change width of .dropdown-menu-toggle to 160px

What does this MR do?

Changes the width of some dropdown toggles from 163px to 160px to match the design spec for input-sm (https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews/).

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8855

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8591#note_131299881

Screenshots

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer