mir 2.30

Mir 2.30 refers to Canonical's Mir 2.30.0 release, a Wayland compositor library covered here mainly for developers, packagers, and maintainers of Mir-based shells. The release makes a Rust compiler a hard requirement for building Mir from source and removes the old MIR_ENABLE_RUST build switch, marking Rust as a permanent part of the toolchain rather than an optional experiment. It also bumps the mirserver ABI to 69 and ships Canonical's first public roadmap, which points toward writing a complete Mir compositor in Rust without C++. For ordinary users of devices running Mir, the build changes have little practical impact.