Run old client test suites against current daemon

[roberth]

Is your feature request related to a problem? Please describe.

We have CI to run the test suite with an older nix-daemon, but not the other way around.
We should test changes to the daemon with an older client.

Describe the solution you'd like

Testing with an older client would be very annoying if we had to add conditionals about the client version in the test suite.
Instead, for master CI, we could run old test suites from the -maintenance branches, and insert our new daemon into it.

This increases the maintenance burden slightly, as changes to the test infrastructure may need to be backported, but this is only occasionally needed, and the added assurance outweighs the time investment. It probably pays off in development effectiveness, as manual testing won't be needed, and mistakes are found sooner.

On the -maintenance branches, we can actually refer to "future" daemon implementations as well, and we should do it. This makes those test infra backports easier, and it could catch mistakes in backports.

Describe alternatives you've considered

Add conditionals on the client version in the test suite, just like we do with the daemon. We had some code that seemed to want to try this, but was never completed. That was removed in #11800. As mentioned, the conditionals would have been very annoying (if not infeasible).

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