muster release v5.9.3

Fixed

• (testing) The muster test start-up sweep terminates only instances whose harness is gone — a concurrent muster test or go test ./internal/testing/ no longer kills a running suite’s instances in #1155 by @teemow

Full Changelog: https://github.com/giantswarm/muster/compare/v5.9.2...v5.9.3