See redis/redis#14826, ETA server 8.8

This is #3034 rebased on main

The new API is marked [SER006]

We also now remove [SER001] (promoting vector-sets to non-experimental), to reflect server doing the same.

Note there's a little noise in here because of a "remove all unused using directives", part of cleaning up after [SER001]. The RespReader changes are for an API added in the V3 version of this API, which is included but /* commented out */.

The CI server is bumped to a build from unstable, which now includes GCRA.