Update CI config

[taiki-e]

• Update actions/checkout action to v4
• Run CI on a regular basis (weekly)
• Use install-action to install cargo-hack and cross for faster & robust installation
• Remove no longer needed argument for cargo fmt
• Remove needless commands
• Remove duplicated cargo doc build
• Use cargo-hack's --rust-version flag for minrust job
  This respects rust-version field in Cargo.toml, so it removes the need to manage MSRV in both the CI file and Cargo.toml.