build: raise minimum build requirements

 * Make gcc 7.4 and boost 1.65.1 hard dependencies, both are
   available out-of-the-box on Ubuntu 18.04
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * If using clang on non-Apple platforms, the minimum supported
   version is now 6.0

Change-Id: I79c84a266741c9a66655da4fd467fe0d8218e8f0
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index 7e19491..f27285b 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