Stable CI

[coord-e]

• remove minimum versions build from ci.yml since it's unstable due to the use of nightly toolchain
• use Cargo.lock and rust-toolchain.toml in CI to:
  • ensure ci.yml is stable (reproducible)
  • improve stability of unstable / flaky checks
• use Swatinem/rust-cache to simplify cache settings
  • it is hard to maintain proper cache key across these various settings manually
• automated update of both files