👷 ci: update Ubuntu runner to 24.04

[ShigureNyako]

Summary

• run the test workflow on the supported Ubuntu 24.04 hosted image
• remove the legacy libtinfo5 bootstrap, which is unavailable in Ubuntu 24.04
• keep the tested LLVM 21.1 / inkwell llvm21-1 pairing and the current actions/checkout@v7 / actions/cache@v6 actions

Why #193 is blocked

#193 only changes the runner from Ubuntu 22.04 to 24.04. Its failing job reaches the Ubuntu 24.04 image and completes checkout, then exits with apt status 100 because libtinfo5 cannot be located. Rustup, LLVM installation, linking, and tests never run, so the observed failure is package availability rather than a Rust, linker/runtime, Actions, or runner-image-EOL failure.

The manual libtinfo5 install was added for an older prebuilt LLVM setup. rcalc now installs LLVM 21 from apt.llvm.org's native Noble repository; its llvm-21-dev package uses Noble's current ncurses development dependency instead. Removing the obsolete prerequisite lets the existing LLVM 21.1 / inkwell 0.9.0 configuration run directly on Ubuntu 24.04.

This incorporates #193's runner update together with its missing compatibility fix. Once merged, #193's one-line change is redundant and can be closed rather than merged separately.

Validation

• actionlint .github/workflows/rcalc-test.yml
• cargo fmt --all -- --check
• cargo test --release — 20 passed
• LLVM_SYS_211_PREFIX=/opt/homebrew/opt/llvm@21 cargo test --features="jit" --release with LLVM 21.1.8 — 24 passed
• all six workflow E2E assertions passed locally with and without JIT
• apt.llvm.org currently publishes the LLVM 21 Noble package set used by this workflow
• TheDan64/inkwell's current LLVM 21.1 job also succeeds on an Ubuntu 24.04 hosted runner with the llvm21-1 feature

The repository has no Dockerfile or other container build definition to update. The pull-request workflow run is the authoritative Ubuntu 24.04 end-to-end validation.