Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
         - { os: ubuntu-latest , ruby: debug }
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1

.github/workflows/nmatrix.yml

@@ -25,7 +25,7 @@ jobs:
         - 2.6
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1

.github/workflows/pycall.yml

@@ -33,7 +33,7 @@ jobs:
         - { os: ubuntu-latest , ruby: debug , python: 3.x , python_architecture: x64 }
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Setup Ruby
       uses: ruby/setup-ruby@v1