Use Ubuntu 24 for GCC 13 coverage

.github/workflows/ci.yml

@@ -63,7 +63,7 @@ jobs:
           - { name: GCC w/ sanitizers, sanitize: yes,
               compiler: gcc-13,    cxxstd: '11,14,17,20',    os: ubuntu-24.04 }
           - { name: Collect coverage, coverage: yes,
-              compiler: gcc-13,    cxxstd: '11,14,17,2a',    os: ubuntu-20.04, install: 'g++-13-multilib', address-model: '32,64' }
+              compiler: gcc-13,    cxxstd: '11,14,17,2a',    os: ubuntu-24.04, install: 'g++-13-multilib', address-model: '32,64' }
 
           # Linux, clang
           - { compiler: clang-3.5, cxxstd: '11',             os: ubuntu-latest, container: 'ubuntu:16.04' }