Bump rustc to nightly-2021-10-21

[MichaelMauderer]

Pull Request Description

Bump rustc to nightly-2021-10-21, upgrade nightly features and fix new lints.

It seems the updated linting detects much more unused code. I left the code that was detected in place and silenced the warnings with a #[allow(dead_code)]. These should be revisited if the code in question is touched again.

[ci no changelog needed]

Checklist

Please include the following checklist in your PR:

• The CHANGELOG.md was updated with the changes introduced in this PR.
• The documentation has been updated if necessary.
• All code conforms to the Rust style guide.
• All code has automatic tests where possible.
• All code has been profiled where possible.
• All code has been manually tested in the IDE.
• All code has been manually tested in the "debug/interface" scene.
• All code has been manually tested by the PR owner against our test scenarios.
• All code has been manually tested by at least one reviewer against our test scenarios.