build: bump uniffi to 0.31.1

[thunderbiscuit]

Just trying to bump uniffi and see what happens with the CI.

See #983.

Notes to the reviewers

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[ItoroD]

Seems safe. Should expect similar for JVM. Aiming for 3.0 or after?

[thunderbiscuit]

Yeah I was thinking of doing the 3.0 on the current uniffi just to ensure we don't break anything for anyone and the bumps goes smoothly for all downstream bindings, but if we have the green light from everyone else (I think dart is the one that we still need to confirm there) then I'd bump on master so that we get it for the 3.1 release.