Bump actions/checkout from 2 to 3.1.0 (#123)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.1.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v3.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Rasmus Lystrøm <[email protected]>

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3.1.0
 
       - name: Setup
         uses: actions/[email protected]
@@ -53,7 +53,7 @@ jobs:
       image: corentinaltepe/doxygen
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3.1.0
 
       - name: Install GraphViz
         run: apk update && apk add graphviz font-bitstream-type1 ghostscript-fonts