Bump actions/checkout from 2 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v3)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Install xmllint
         run: sudo apt-get install libxml2-utils
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: xmllint the schema
         run: xmllint --schema http://www.w3.org/2001/XMLSchema.xsd --noout *.xsd