chore: update GitHub Actions CI + remove redundant job

[striezel]

The following updates are performed:

• update actions/checkout to v3
• replace unmaintained actions-rs/toolchain by dtolnay/rust-toolchain
• replace unmaintained actions-rs/cargo by direct invocations of cargo

Still using the outdated / unmaintained actions will generate several warnings in CI runs, for example in https://github.com/alexkirsz/dispatch/actions/runs/4903327790:

Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16: actions/checkout@v2, actions-rs/toolchain@v1, actions-rs/cargo@v1. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

The PR will get rid of those warnings.

Futhermore, the redundant combo job is removed, because just does what the the two previous jobs ('ClippyandFormat`) in the same workflow do, so there's no need to keep it.

[alexkirsz]

Thanks for the contribution!