GHA: Update to actions/checkout@v3

v2 is deprecated.

Signed-off-by: Takuro Ashie <[email protected]>

Author: ashie

.github/workflows/linux.yml

@@ -17,7 +17,7 @@ jobs:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}