Update actions/checkout requirement to v2.3.4

Updates the requirements on [actions/checkout](https://github.com/actions/checkout) to permit the latest version.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/commits/5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/documentation.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2.3.4
       with:
         fetch-depth: 50
     - name: Run website_build.sh

.github/workflows/epochs.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2.3.4
     - name: Run epochs_update.sh
       # Use a conditional step instead of a conditional job to work around #20700.
       if: github.repository == 'web-platform-tests/wpt'

.github/workflows/manifest.yml

@@ -15,7 +15,7 @@ jobs:
       with:
         python-version: '3.x'
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2.3.4
       with:
         fetch-depth: 50
     - name: Install dependencies

.github/workflows/regen_certs.yml

@@ -13,7 +13,7 @@ jobs:
       with:
         python-version: '3.x'
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v2.3.4
     - name: Regenerate certs
       # Use a conditional step instead of a conditional job to work around #20700.
       if: github.repository == 'web-platform-tests/wpt'