Update the mock-server-netty dep; specify the logback dep

Some of our tests use mock-server-netty 5.4.1. Full WF uses 5.6.1. Moving to 5.6.1 is good because:

1) It's good to be aligned with full, just because.
2) 5.4.1 brings in some transitive deps that we get vulnerability warnings about, which is annoying to deal with just for a test dep.

However, moving to 5.6.1 means there's no longer a mock-server-netty dep pulling in ch.qos.logback:logback-classic and logback-core. And it seems a couple tests in testsuite/embedded require at least one of those deps, as I guess they are testing how embedded works when logback is the embedding app's logging impl. So the fix there is to explicitly declare the logback dep.