OptaPlanner Micrometer monitoring: rename optaplanner.solver.solve-length to optaplanner.solver.solve.duration

As discussed here:
https://kie.zulipchat.com/#narrow/stream/235756-optaplanner-dev/topic/Micrometer.3A.20solve-length.20metric.20name

Summary: we misnamed one of our micrometer properties and we don't want that misname to escape into the wild to our customers.
Because we didn't release documentation or announcements for it yet - and micrometer support is new in PAM 7.12 for optaplanner, it's still safe to do this now.

Backport PR that is waiting for triaging to be merged: https://github.com/kiegroup/optaplanner/pull/1562