Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...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-pr.yaml

@@ -29,7 +29,7 @@ jobs:
         uses: frequenz-floss/[email protected]
 
       - name: Fetch sources
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true
 

.github/workflows/ci.yaml

@@ -79,7 +79,7 @@ jobs:
         uses: frequenz-floss/[email protected]
 
       - name: Fetch sources
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true
 
@@ -177,7 +177,7 @@ jobs:
         uses: frequenz-floss/[email protected]
 
       - name: Fetch sources
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true
 
@@ -213,7 +213,7 @@ jobs:
         uses: frequenz-floss/[email protected]
 
       - name: Fetch sources
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true