Bump actions/checkout from 2 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
         os: [ubuntu-latest]
         compiler: [gcc, g++, clang, gcc-10]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     - name: Compile Library
       env:
         CCFLAGS: "-fsanitize=undefined"
@@ -27,7 +27,7 @@ jobs:
           os: [ubuntu-latest]
           compiler: [gcc]
       steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - name: Compile Library
         env:
           CCFLAGS: "-fsanitize=undefined"
@@ -48,7 +48,7 @@ jobs:
         os: [ubuntu-latest]
         compiler: [gcc]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     # The following should be moved into a seperate action for only pull requests, etc.
     # where you it posts the information into the PR as a comment (optionally?)
     - name: Install cppcheck