Build/Test Tools: Test against `7.0-RC3`.

This updates the upgrade testing workflows to test against RC3. After 7.0 final release, this should be updated to test `7.0` proper.

Fixes #64966.

git-svn-id: https://develop.svn.wordpress.org/trunk@62358 602fd350-edb4-49c9-b593-d223f7449a82