Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
           components: clippy
 
       - name: "Cache cargo registry"
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.cargo/registry
@@ -62,7 +62,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: "Cache cargo registry"
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.cargo/registry
@@ -87,7 +87,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: "Cache cargo registry"
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.cargo/registry