Update jthread source

Not much has changed in jthread in 5 years, but it's still slightly backwards-incompatible with the old version. Just a namespace wrapper and some internal constants refactored.

This updates the jthread sources to 1.3.1 (the latest version) and has the side effect of fixing linking to system jthread libraries of that version. Any version of jthread 1.3.0 or later is now supported.