Update to UniFFI 0.31

[mhammond]

which hasn't actually been released yet :) But @bendk and I are starting to plan for the release and bumping m-c (bug 2007857), app-services (wip branch) and glean (my hacky wip poc branch).

I'm opening this to make y'all aware of this, see if you have any concerns etc, and to get an initial idea of when you might be willing and able to take the cleaned up PR?

[badboy]

I have a branch running against UniFFI main. I can push that.