demo: switch to bevyengine/bevy

[johanhelsing]

We're currently using my own fork: https://github.com/johanhelsing/bevy, which is not ideal, we should just use plain bevy main branch.

There are some problems in the way, however. bevy_webgl2 does not compile with latest bevy, and bevy_ggrs depends on patches not present in bevy 0.5.

One option is to simply wait for bevy 0.6 and bevy_webgl to update.

Another is to fix the compatibility issues in bevy_webgl and use a patched version of that. Perhaps even ask if it could be hosted in the official bevy_webgl2 repo.

[johanhelsing]

mrk-its/bevy_webgl2#49

[johanhelsing]

Now tracking upstream main of all git dependencies