build: align minimum clang dependency with ndn-cxx

ndn-cxx requires clang 6 or later, so there's no point
supporting older versions in ndn-tools

Change-Id: I7be683abd4fce12c85187c0b5abf2ee288bbe908
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index f94c307..a5b4c30 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -21,7 +21,7 @@
       WAF_JOBS: 3
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Install dependencies
         run: |
           case ${RUNNER_OS} in