Bump minimum clang version to 7.0.1

std::variant is broken in clang < 7.0.1 with libstdc++
see: https://github.com/llvm/llvm-project/issues/32569

.github/workflows/build.yml

@@ -76,21 +76,21 @@ jobs:
           ./bin/minetest --run-unittests
 
   # Older clang version (should be close to our minimum supported version)
-  clang_6_0:
+  clang_7:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - name: Install deps
         run: |
           source ./util/ci/common.sh
-          install_linux_deps clang-6.0 valgrind
+          install_linux_deps clang-7 valgrind
 
       - name: Build
         run: |
           ./util/ci/build.sh
         env:
-          CC: clang-6.0
-          CXX: clang++-6.0
+          CC: clang-7
+          CXX: clang++-7
 
       - name: Unittest
         run: |