[github-actions] remove clang-6,7,8 from build check (#7969)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d158eea..47d4fa2 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -216,7 +216,7 @@
     strategy:
       fail-fast: false
       matrix:
-        clang_ver: ["6.0", "7", "8", "9", "10", "11", "12", "13"]
+        clang_ver: ["9", "10", "11", "12", "13"]
     env:
       CC: clang-${{ matrix.clang_ver }}
       CXX: clang++-${{ matrix.clang_ver }}
@@ -246,7 +246,7 @@
     strategy:
       fail-fast: false
       matrix:
-        clang_ver: ["6.0", "7", "8", "9", "10", "11", "12", "13"]
+        clang_ver: ["9", "10", "11", "12", "13"]
     env:
       CC: clang-${{ matrix.clang_ver }}
       CXX: clang++-${{ matrix.clang_ver }}