Update CI image to Fedora 33
Fedora 33 is currently scheduled to be released on 2020-10-20. We should update the CI images soon after, so that we are running against the latest
- The Meson downgrade can be removed when we do this upgrade.
- Building IWYU will be difficult unless IWYU has released a version of their source code that matches Clang 11 by that time, or you might try building from IWYU master branch.
- If the upgrade includes g-i 1.66.1, then there are some skipped Regress tests that can be enabled.