Update actions/cache to @v4

Author: iamalwaysuncomfortable

.github/workflows/staging-website.yml

@@ -22,7 +22,7 @@ jobs:
           override: true
           components: rustfmt, rust-src
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry

.github/workflows/test-website.yml

@@ -19,7 +19,7 @@ jobs:
           override: true
           components: rustfmt, rust-src
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry

.github/workflows/website.yml

@@ -22,7 +22,7 @@ jobs:
           override: true
           components: rustfmt, rust-src
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry