Update GitHub Actions CI

The following updates are performed:
* update actions/cache to v3
* update actions/checkout to v3

.github/workflows/doc.yml

@@ -29,7 +29,7 @@ jobs:
     concurrency: doc-${{ matrix.crates }}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           submodules: true
       - name: Install Rust targets

.github/workflows/rust.yml

@@ -22,11 +22,11 @@ jobs:
       id-token: write
       contents: read
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           submodules: true
       - name: Cache multiple paths
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: |
             ~/.cargo/bin/
@@ -44,11 +44,11 @@ jobs:
   build-stable:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           submodules: true
       - name: Cache multiple paths
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: |
             ~/.cargo/bin/
@@ -67,7 +67,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Test boot
         working-directory: ./embassy-boot/boot