GHA: Use Clang 7 as lowest version for libc++ test

The libc++ package for clang-6.0 does not exist on Ubuntu 20.04

.github/workflows/ci.yml

@@ -74,7 +74,7 @@ jobs:
           - { compiler: clang-18,  cxxstd: '11,14,17,20,2b', os: ubuntu-latest, container: 'ubuntu:24.04' }
 
           # libc++
-          - { compiler: clang-6.0, cxxstd: '03,11,14',       os: ubuntu-20.04, stdlib: libc++, install: 'clang-6.0 libc++-dev libc++abi-dev' }
+          - { compiler: clang-7,   cxxstd: '03,11,14',       os: ubuntu-20.04, stdlib: libc++, install: 'clang-7 libc++-7-dev libc++abi-7-dev' }
           - { compiler: clang-12,  cxxstd: '03,11,14,17,20', os: ubuntu-20.04, stdlib: libc++, install: 'clang-12 libc++-12-dev libc++abi-12-dev' }
           - { name: Clang w/ sanitizers, sanitize: yes,
               compiler: clang-12,  cxxstd: '03,11,14,17,20', os: ubuntu-20.04, stdlib: libc++, install: 'clang-12 libc++-12-dev libc++abi-12-dev' }