Seamless getdown updates between j8 based Jalview 2.11.x and j11 based Jalviews

getdown channels with different JVM requirements cause problems for Jalview 2.11.x installations - when Jalview's target platform becomes Java 11+, it should still be possible to smoothly launch older jalview versions via a JVL file. JAL-3844 describes the current issue - that the getdown launcher doesn't use the updated JVM installation to launch the getdown channel, so one has to launch the launcher once again for it to work (but sometimes this still fails)