ci: Bump actions/cache to v5

https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Author: archessmn

.github/workflows/backend.yml

@@ -11,7 +11,7 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Cache build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/.cargo/bin/**

.github/workflows/format.yml

@@ -11,7 +11,7 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Cache build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/.cargo/bin/**