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/format.yml

@@ -15,7 +15,7 @@ jobs:
     pre-commit:
         runs-on: ubuntu-latest
         steps:
-        -   uses: actions/checkout@v4
+        -   uses: actions/checkout@v6
             with:
                 fetch-depth: 0
         -   uses: actions/setup-python@v6

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
         runs-on: ubuntu-latest
 
         steps:
-        -   uses: actions/checkout@v4
+        -   uses: actions/checkout@v6
             with:
                 fetch-depth: 0
 

.github/workflows/update_precommit_hooks.yml

@@ -32,7 +32,7 @@ jobs:
         runs-on: ubuntu-latest
         steps:
         -   name: Checkout
-            uses: actions/checkout@v4
+            uses: actions/checkout@v6
         -   name: Set up Python
             uses: actions/setup-python@v6
             with:

.github/workflows/validation.yml

@@ -16,7 +16,7 @@ jobs:
     validate_cff:
         runs-on: ubuntu-latest
         steps:
-        -   uses: actions/checkout@v4
+        -   uses: actions/checkout@v6
         -   name: Check whether the citation metadata from CITATION.cff is valid
             uses: citation-file-format/cffconvert-github-action@2.0.0
             with: