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.

.github/workflows/main.yml

@@ -53,7 +53,7 @@ jobs:
           rust: nightly-gnu
           other: i686-pc-windows-gnu
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Update Rustup (temporary workaround)
       run: rustup self update
       shell: bash
@@ -103,14 +103,14 @@ jobs:
   resolver:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - run: rustup update stable && rustup default stable
     - run: cargo test --manifest-path crates/resolver-tests/Cargo.toml
 
   build_std:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - run: rustup update nightly && rustup default nightly
     - run: rustup component add rust-src
     - run: cargo build
@@ -120,7 +120,7 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - run: rustup update nightly && rustup default nightly
     - run: rustup update stable
     - run: rustup component add rust-docs

src/doc/src/guide/continuous-integration.md

@@ -46,7 +46,7 @@ jobs:
           - beta
           - nightly
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}
       - run: cargo build --verbose
       - run: cargo test --verbose