java-idp-jetty-base: persistTempDirectory in idp.xml for Jetty 12

[Rod Widdowson]

> Will this Jetty 12 release become the minimum supported version straight away, or only in January when Jetty 11 is formally EOL?

I'll let Scott answer that, but I'll take advantage of this thread to remind everyone that, starting with Jetty12, the installer
does not provide back channel support.

So, no matter what their life cycle plans, installations need to make sure that they have stopped using backchannel.

Remember, too, that you need to allow for a full metadata recycle refresh cycle to elapse before you can make any changes mainline.

Rod