Use `GlKeysetPagination` in `app/assets/javascripts/diffs/components/app.vue`

Purpose

We are revising our pagination guidance to make better use of our existing components. This means switching the usage of GlPagination to use GlKeysetPagination in situations where keyset pagination is more appropriate.

Proposal

• Use GlKeysetPagination instead of GlPagination in compact mode