Prepare v1.14 stable branch

Notes:
- MLH configuration dropped jenkins flake correlator since main / v1.14
  doesn't use jenkins any more.
- v1.13 stable branch dropped Go caching in the CI builds, but this time
  around I kept it. I suspect that this caching was only dropped from
  v1.13 CI builds because at the time I had copied the old v1.12 version
  of the workflow into the v1.13 tree in order to do this
  initialization. This time I based the change on main.

Open questions:
- There's quite a few workflows that use the default branch for checkout via
  `github.event.repository.default_branch`, but it's not clear to me how
  we should customize such workflows for the v1.14 stable branch.t sh

Signed-off-by: Joe Stringer <joe@cilium.io>