Update GitHub Actions actions/checkout@v2 to v3

The v2 implementation uses Node 12, which is end-of-life on April 30, 2022.
See https://nodejs.org/en/about/releases/. Update to v3, which is based on
Node 16 whose support lasts until April 30, 2024.

Author: dtolnay

.github/workflows/integration.yml

@@ -69,7 +69,7 @@ jobs:
 
     steps:
     - name: checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
       # Run build
     - name: install rustup

.github/workflows/linux.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
     - name: checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
       # Run build
     - name: install rustup

.github/workflows/mac.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
     - name: checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
       # Run build
     - name: install rustup

.github/workflows/rustdoc_check.yml

@@ -11,7 +11,7 @@ jobs:
     name: rustdoc check
     steps:
     - name: checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     - name: install rustup
       run: |

.github/workflows/upload-assets.yml

@@ -31,7 +31,7 @@ jobs:
             target: x86_64-pc-windows-msvc
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
         # Run build
       - name: install rustup

.github/workflows/windows.yml

@@ -33,7 +33,7 @@ jobs:
     - name: disable git eol translation
       run: git config --global core.autocrlf false
     - name: checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
       # Run build
     - name: Install Rustup using win.rustup.rs