github: migrate to v3 action

Using v2 action are deprecated.

See https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

.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 }}