feat(GILoadingIcon): change default size

This MR changes the default size of GILoadingIcon from "sm" (16x16) to "md" (24x24).

For context, here are related MRs / Issues:

• gitlab-org/gitlab-services/design.gitlab.com#270 (comment 191389919)
• !633 (comment 194180079)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/62609

MR that prepares the migration by adding a sm-size to all instances of gl-loading-icon without a size prop:

• gitlab!65006 (merged) (merged)

Note: This MR is related to gitlab#29847 (closed) (including a change to the new default size)