Accessibility: Improve color contrast for the blue links `:hover` state.

For a number of years, WordPress has been using a `#00a0d2` blue shade for the links `:hover` state. This blue shade doesn't have a sufficient color contrast with the various (too many) background colors used in the admin interface.

The new `#006799` blue shade is part of the official WordPress color palette and does have a sufficient color contrast with most of the admin backgrounds.

Props ryokuhi, audrasjb, joedolson, mapk.
See #47682.

Built from https://develop.svn.wordpress.org/trunk@48368


git-svn-id: https://core.svn.wordpress.org/trunk@48137 1a063a9b-81f0-0310-95a4-ce76da25c4cd