Update actions/checkout requirement to v2.3.4 (restsharp#1580)

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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/build-dev.yml

@@ -16,7 +16,7 @@ jobs:
 #    runs-on: ubuntu-latest
 #
 #    steps:
-#      - uses: actions/checkout@v1
+#      - uses: actions/checkout@v2.3.4
 #      - uses: actions/setup-dotnet@v1
 #        with:
 #          dotnet-version: '3.1.201'
@@ -28,7 +28,7 @@ jobs:
     runs-on: windows-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.4
 
     - name: Run tests on Windows for all targets
       run: dotnet test -c Release 
@@ -38,7 +38,7 @@ jobs:
     needs: [test-win] #,test-netcore-linux]
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.4
 
     - name: Create and push NuGet package
       run: |

.github/workflows/codeql-analysis.yml

@@ -23,7 +23,7 @@ jobs:
 
       - name: Checkout repository
         id: checkout_repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v2.3.4
 
       - name: Initialize CodeQL
         id: init_codeql

.github/workflows/pull-request.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
       - uses: actions/setup-dotnet@v1.8.0
         with:
           dotnet-version: '3.1.201'
@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
 
       - name: Vuepress
         run: |