chore: Update caching keys in CI workflow to reflect Node.js version 20

### Summary of Changes
- Modified caching keys in the CI workflow configuration to use Node.js version `20` instead of `18`, ensuring proper cache management for dependencies.

### Expected Results
- Improved efficiency in dependency caching during CI runs, aligning with the recent Node.js version upgrade.

.gitea/workflows/ci.yml

@@ -222,9 +222,9 @@ jobs:
           ~/.npm
           node_modules
           ~/.cache/node-gyp
-        key: npm-${{ runner.os }}-18-${{ hashFiles('frontend/package-lock.json') }}
+        key: npm-${{ runner.os }}-20-${{ hashFiles('frontend/package-lock.json') }}
         restore-keys: |
-          npm-${{ runner.os }}-18-
+          npm-${{ runner.os }}-20-
           npm-${{ runner.os }}-
           npm-
         fail-on-cache-miss: false