ci (check-whitespace): move to actions/checkout@v3

Get rid of deprecation warnings in the CI runs.  Also gets the latest
security patches.

Signed-off-by: Chris. Webster <chris@webstech.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/check-whitespace.yml

@@ -13,7 +13,7 @@ jobs:
   check-whitespace:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
         fetch-depth: 0