Add rust-toolchain file

[IceSentry]

Objective

• When bisecting a bug we end up using older versions of bevy that don't always compile with the most recent compiler
• We use cutting edge features and not everyone has the latest compiler version installed and they end up seeing errors that might not be easy to understand

Solution

• Add a rust-toolchain file to document which compiler version is used for the current state of the repo
• This way when using git bisect we don't have to guess at which compiler version we should be using
• This means we will have to make a new PR to update the file every time there's a new rust version or at least every time we use a feature from a newer rust version
• Unfortunately, this caused a few issues with CI so I had to add rustup override steps to a few of the CI actions that were expecting a specific rust version or component. This way CI essentially doesn't have to care about the toolchain file.

Testing

• I tested a local build and it used the expected compiler version

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[mockersf]

could you set up dependabot to update it? https://github.blog/changelog/2025-08-19-dependabot-now-supports-rust-toolchain-updates/