Bump actions/cache from 3 to 4

Bumps [actions/cache](https://github.com/actions/cache) from 3 to 4.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v3...v4)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/backend.yml

@@ -31,7 +31,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo registry, index and build directory
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry
@@ -68,7 +68,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo registry, index and build directory
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry

.github/workflows/time-cache.yml

@@ -31,7 +31,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo registry, index and build directory
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry
@@ -68,7 +68,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo registry, index and build directory
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry