Various CI jobs have been updated to use modern OS images, drop
dead code, and remove redundant jobs. Support for Python 2 in CI
has been removed, and the linux-TEST-vars job has been renamed to
linux-exotic.
* ps/ci-housekeeping-and-modernizations:
ci: improve reftable test coverage
ci: drop now-dead Python 2 coverage
ci: switch away from EOL'd Ubuntu version in linux-exotic
ci: rename linux-TEST-vars job
ci: switch away from unsupported i386/ubuntu image
ci: drop unused "linux-clang" logic
ci: fix "fedora-breaking-changes-meson" job
t5004: skip SHA-1-only test in SHA-256 repository