ci: run GCC jobs on Ubuntu 24.04

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Summary

• migrate the six GCC Linux CI jobs from ubuntu-20.04 to ubuntu-24.04
• remove the six Clang build jobs and the Clang unit-test job
• leave the GCC job build commands unchanged

Why

GitHub retired the hosted Ubuntu 20.04 runner image. The Linux jobs could no longer acquire a matching runner, waited for 24 hours, and were then cancelled without executing any steps.

The Linux CI scope is now intentionally limited to GCC.

Impact

The retained GCC jobs can be scheduled on a supported GitHub-hosted runner. Clang builds and the Clang unit-test job are no longer run by this workflow.

Validation

• parsed .github/workflows/ci-linux.yml successfully as YAML
• git diff --check passes
• verified that the workflow contains exactly six jobs, all prefixed with gcc-

Known failures exposed by the migration

• the standard GCC Bazel job still uses the repository-wide C++11 Bazel setting, while current code requires a newer C++ standard
• the all-options GCC Bazel job uses a stale Boost download URL whose content no longer matches the pinned checksum
• the macOS Homebrew failure is separate and outside the scope of this PR