CI: bump actions/checkout from v3/v4 to v6 (#494)

## Why

With v3, we have the following deprecation notices:

```
The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/
```
from https://github.com/collectiveidea/awesome_nested_set/actions/runs/11509088498.

See https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

<blockquote>

Node16 has been out of support since [September 2023](https://github.com/nodejs/Release/#end-of-life-releases). As a result we have started the deprecation process of Node16 for GitHub Actions. We plan to migrate all actions to run on Node20 by Spring 2024.
Following on from our [warning in workflows using Node16](https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/) we will start enforcing the use of Node20 rather than Node16 on the 3rd of June.

</blockquote>

## What

Bump [actions/checkout](https://github.com/actions/checkout) from v3 (and v4) to v6.

---------

Signed-off-by: Takuya Noguchi <[email protected]>
Co-authored-by: Philip Arndt <[email protected]>

Author: tnir

.github/workflows/ci.yml

@@ -86,7 +86,7 @@ jobs:
       BUNDLE_PATH_RELATIVE_TO_CWD: true
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with:
@@ -191,7 +191,7 @@ jobs:
           --health-retries=3
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with:
@@ -298,7 +298,7 @@ jobs:
           --health-retries 5
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with:

.github/workflows/publish_gem.yml

@@ -17,7 +17,7 @@ jobs:
 
     steps:
       # Set up
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: