Cannot pip install by default any more

Since Lunar or maybe Kinetic, "pip install" won't work on the system and requires a virtualenv unless you use --break-system-packages. But I don't see this release noted in the past.

People will notice and see it as a breaking change, so we should release note it. For example, Dockerfiles that use "pip install" will break and users will either have to use --break-system-packages or arrange virtualenvs, even though within a Docker environment, if it works it's not usually an issue.

We should probably document this and have recommended answer for various use cases.