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/check_md_links.yml

@@ -17,7 +17,7 @@ jobs:
     markdown-link-check:
         runs-on: ubuntu-latest
         steps:
-        -   uses: actions/checkout@v4
+        -   uses: actions/checkout@v6
         -   uses: tcort/github-action-markdown-link-check@v1
             with:
                 use-quiet-mode: yes

.github/workflows/test_and_coverage.yml

@@ -21,7 +21,7 @@ jobs:
         runs-on: ubuntu-latest
         steps:
         -   name: Clone repo
-            uses: actions/checkout@v4
+            uses: actions/checkout@v6
             with:
                 submodules: true
         -   name: Set up Python

.github/workflows/update_spreadsheets.yml

@@ -22,7 +22,7 @@ jobs:
         if: github.repository_owner == 'ohbm'
         steps:
         -   name: Checkout repository
-            uses: actions/checkout@v4
+            uses: actions/checkout@v6
             with:
                 submodules: true
         -   name: Set up Python

.github/workflows/validate.yml

@@ -21,7 +21,7 @@ jobs:
     validate:
         runs-on: ubuntu-latest
         steps:
-        -   uses: actions/checkout@v4
+        -   uses: actions/checkout@v6
             with:
                 submodules: true
         -   name: Setup Node