MM-23261 plugin stderr debug logs (#14166)

* explicitly assert panic as error log

* Revert "[MM-18150] plugin panic trace should not be lost (#13559)"

This reverts commit 5d928b4f94, while leaving the unit tests intact
and now asserting debug logs instead.

* missing license header

plugin/supervisor.go

@@ -63,7 +63,6 @@ func newSupervisor(pluginInfo *model.BundleInfo, apiImpl API, parentLogger *mlog
 		HandshakeConfig: handshake,
 		Plugins:         pluginMap,
 		Cmd:             cmd,
-		Stderr:          wrappedLogger.With(mlog.String("source", "plugin_stderr_panic")).StdErrPanicLogWriter(),
 		SyncStdout:      wrappedLogger.With(mlog.String("source", "plugin_stdout")).StdLogWriter(),
 		SyncStderr:      wrappedLogger.With(mlog.String("source", "plugin_stderr")).StdLogWriter(),
 		Logger:          hclogAdaptedLogger,