Upgrade: bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/testing.yml

@@ -10,7 +10,7 @@ jobs:
     name: Test run action (triggers warnings)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         path: example
@@ -21,7 +21,7 @@ jobs:
     name: Test run action (valid)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         path: example_valid
@@ -30,7 +30,7 @@ jobs:
     name: Test run action (valid with custom Flake8)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - run: pip install flake8==3.8.0
     - uses: ./
       with:
@@ -40,7 +40,7 @@ jobs:
     name: Test run action (valid with Flake 3.8.0)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         path: example_valid
@@ -50,7 +50,7 @@ jobs:
     name: Test run action (valid with Python 3.9)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v5
       with:
         python-version: 3.9
@@ -62,7 +62,7 @@ jobs:
     name: Test run action (plugins)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         plugins: flake8-bugbear
@@ -73,7 +73,7 @@ jobs:
     name: Test run action (classes)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         plugins: flake8-bugbear
@@ -86,7 +86,7 @@ jobs:
     name: Test run action (extra arguments)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         path: example_length
@@ -96,7 +96,7 @@ jobs:
     name: Test run action (working dir)
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: ./
       with:
         working_directory: example_valid
@@ -105,7 +105,7 @@ jobs:
     name: Check for spelling errors
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: codespell-project/actions-codespell@master
       with:
         check_filenames: true