Add `rust-version` field to `bevy_mikktspace`

[BD103]

bevy_mikktspace documents that it requires at least Rust 1.52.1.

https://github.com/bevyengine/bevy/blob/ff8a9b2a64d4d34ceb359edbf80298dcd5f9c9fb/crates/bevy_mikktspace/README.md?plain=1#L13

This should be codified by adding the rust-version field to Cargo.toml.

[BD103]

Bonus points if you also check if any other crates do not specify a rust-version. :) (Most should say rust-version.workspace = true!)

[Brezak]

bevy_mikktspace is using the 2021 Rust edition. This makes it's minimum supported version Rust 1.56.0. The documentation will need to be updated.

[BD103]

Bonus points if you also check if any other crates do not specify a rust-version. :) (Most should say rust-version.workspace = true!)

Turns out workspace-inherited properties are quite incompatible with our compile fail tests. If that were fixed, a lot of duplicated metadata could be merged...