Use Cargo workspace features for internal dependency versions

Currently all of the Bevy sub-packages which depend on other parts of Bevy use explicit version numbers. This means that whenever a version of a package is bumped, you have to change the version number in every Cargo.toml that references it.

It would be much less work to use Cargo's workspace versioning feature:

```
bevy_app = { workspace = true }
```
This will use the version number specified in the top-level Cargo.toml (in the section `workspace.dependencies`).

Doing this means that the package version for each crate only needs to be specified in two places: in the package itself, and in the top-level `workspace.dependencies`. This will making bumping the versions much easier.