Upgrade to Jetty 12.0.12

[snicoll]

[snicoll]

Jetty has deprecated WebInfConfiguration.getCanonicalNameForWebAppTmpDir that we use to source the temporary directory the embedded Jetty server uses. I am not sure what to do with the deprecation.

[wilkinsona]

I think we can switch to the new getCanonicalNameForTmpDir method that was added to WebAppContext in 12.0.12. That would make us incompatible with Jetty 12.0.11 and earlier. If we want to maintain some backwards compatibility, we could fall back to WebInfConfiguration.getCanonicalNameForWebAppTmpDir if a call to getCanonicalNameForTmpDir results in a NoSuchMethodError.