Bump matrix-sdk dependency to 0.8.

[dstu]

This accommodates recent protocol changes and addresses #378.

[vkleen]

It looks like a flake.nix needs to be updated as well like in #214.

[bnjbvr]

Hi! Rust SDK maintainer over here. Thanks for doing this PR, it's interesting for us to see how many breaking changes we've introduced, and this helped me realized a doc comment was wrong about compute_display_name(). I made a few suggestions below, but generally the code looks good to me.

[ulyssa]

The changes here look good to me! I'm going to try this out locally a little bit first, but I think the only thing left that this needs is a run of cargo +nightly fmt to fix CI.

[dstu]

Hm, it looks like matrix-sdk 0.9 just got released (in the last 20 days). Happy to keep get this merged and then work on migrating to 0.9. But, if it's better to jump straight to 0.9, then I can transition to that.