CI: Update to actions/checkout@v3

No functional change, but we should keep this current.
diff --git a/.github/workflows/coverity.yml b/.github/workflows/coverity.yml
index 0c4b2d0..e208c8c 100644
--- a/.github/workflows/coverity.yml
+++ b/.github/workflows/coverity.yml
@@ -24,7 +24,7 @@
       # parameters at whitespaces, without taking quoting into account.
       COVERITY_SCAN_BUILD_COMMAND: "make CC=clang TARGET=linux-glibc USE_ZLIB=1 USE_PCRE=1 USE_PCRE_JIT=1 USE_LUA=1 USE_OPENSSL=1 USE_QUIC=1 USE_SYSTEMD=1 USE_WURFL=1 WURFL_INC=addons/wurfl/dummy WURFL_LIB=addons/wurfl/dummy USE_DEVICEATLAS=1 DEVICEATLAS_SRC=addons/deviceatlas/dummy USE_51DEGREES=1 51DEGREES_SRC=addons/51degrees/dummy/pattern ADDLIB=\"-Wl,-rpath,$HOME/opt/lib/\" SSL_LIB=${HOME}/opt/lib SSL_INC=${HOME}/opt/include DEBUG+=-DDEBUG_STRICT=1 DEBUG+=-DDEBUG_USE_ABORT=1"
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install apt dependencies
       run: |
         sudo apt-get update