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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build-packages.yml

@@ -15,7 +15,7 @@ jobs:
       INSPIRCD_PACKAGES: ${{ matrix.package }} html
       INSPIRCD_REVISION: ${{ github.sha }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Retrieve latest release
         run: |
           echo INSPIRCD_VERSION=$(curl https://api.github.com/repos/inspircd/inspircd/releases | jq --raw-output 'map(select((.draft or .prerelease | not) and (.tag_name | startswith("v4.")))) | first .tag_name | .[1:]') >> $GITHUB_ENV