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] <support@github.com>

Author: dependabot[bot]

.github/workflows/firebase-hosting-merge.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: npm install -g yarn && yarn install --frozen-lockfile && yarn build
       - name: Substitute M-Lab project placeholder
         run: sed -i 's/MLAB_PROJECT_PLACEHOLDER/mlab-oti/g' app/measure/measure.js

.github/workflows/firebase-hosting-pull-request.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: npm install -g yarn && yarn install --frozen-lockfile && yarn build
       - uses: FirebaseExtended/action-hosting-deploy@v0
         with: