CI: Update to actions/checkout@v3

No functional change, but we should keep this current.
diff --git a/.github/workflows/compliance.yml b/.github/workflows/compliance.yml
index 148ea28..1105abb 100644
--- a/.github/workflows/compliance.yml
+++ b/.github/workflows/compliance.yml
@@ -21,7 +21,7 @@
     env:
       H2SPEC_VERSION: '2.6.0'
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install h2spec
       run: |
         curl -fsSL https://github.com/summerwind/h2spec/releases/download/v${H2SPEC_VERSION}/h2spec_linux_amd64.tar.gz -o h2spec.tar.gz