Update GitHub Actions to use non-deprecated versions

- Update actions/upload-artifact from v3 to v4
- Update actions/cache from v3 to v4

Fixes deprecation warnings in CI workflow.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jwaldrip

.github/workflows/ci.yml

@@ -138,7 +138,7 @@ jobs:
         run: crystal docs
 
       - name: Upload documentation
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: documentation
           path: docs/

.github/workflows/test.yml

@@ -28,7 +28,7 @@ jobs:
           crystal: ${{ matrix.crystal }}
 
       - name: Cache shards
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: |
             lib