ci: Use latest actions/checkout@v4 for CI

It fixes the following warning:

  The following actions uses node12 which is deprecated and will be
  forced to run on node16: actions/checkout@v2. For more info:
  https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -12,7 +12,7 @@ jobs:
         os: [ 'ubuntu-latest' ]
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}

.github/workflows/windows.yml

@@ -12,7 +12,7 @@ jobs:
         os: [ 'windows-latest' ]
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}