Bump node.js requirement to 20.
According to https://github.com/matrix-org/matrix-js-sdk?tab=readme-ov-file#supported-platforms, we only support the latest LTS release (which is currenly 20, per https://github.com/nodejs/release#release-schedule), so this should be safe.
@richvdh looks like this will need downstream changes first, as that CI still runs under Node 18
Hopefully https://github.com/matrix-org/matrix-react-sdk/pull/12726 will fix the downstream CI
... and https://github.com/matrix-org/complement-crypto/pull/102
Oh come on. Test flake due to https://github.com/matrix-org/complement-crypto/issues/9