Update shape of skeleton loader on Releases and Release pages

What does this MR do?

Updates the shape of the skeleton loader on the Releases page (and the individual Release page) as described in #201959 (closed).

The shape of the loader is based on the Figma mockups in gitlab-org/gitlab-services/design.gitlab.com#695 (closed). The shape has been modified slightly in order to more exactly match the shape of the real release block element (see the comparison below).

Screenshots

Old skeleton loader shape

Releases page

Individual Release page

Real release block overlaid on skeleton loader

(For demonstration purposes only - this will never occur in real life.)

GIF