Adjust spacing of disclosure dropdown to match Figma

Adjust left and right padding of src/components/base/new_dropdowns/disclosure/disclosure_dropdown.vue. This was suggested by @pedroms in a comment on Adjust styling of md editor header (gitlab!137294 - merged).

Regarding the size of the “more options” button, I just realized that its padding is inconsistent with the Figma library — see small dropdown icon button in GitLab UI vs Figma. Using the Figma specs will ensure that small icon buttons and small dropdown icon buttons have the same padding.

cf. gitlab!137294 (comment 1673694988), gitlab#433291 (comment 1677895869)

"More options" disclosure in Rich Text editor	icon disclosure button in [Figma](https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/%F0%9F%93%99-Component-library?type=design&node-id=2956-3382&mode=design&t=IKgYueSU9tx7yrEO-4