Bump minimum tested gcc version to 8

Author: mat007

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
       matrix:
         include:
           # Linux, gcc
-          - { compiler: gcc-7, cxxstd: '14,17,20', boostBranch: boost-1.86.0, os: ubuntu-20.04 }
+          - { compiler: gcc-8, cxxstd: '14,17,20', boostBranch: boost-1.86.0, os: ubuntu-20.04 }
           - { compiler: gcc-14, cxxstd: '14,17,20', boostBranch: boost-1.86.0, os: ubuntu-24.04 }
           - { compiler: gcc-14, cxxstd: '14,17,20', boostBranch: master, os: ubuntu-24.04 }