ci: update Codecov integration to v5 with token authentication

Update code coverage upload to use codecov-action@v5 with proper
authentication and repository identification.

Changes:
- Upgrade from codecov-action@v4 to v5
- Add CODECOV_TOKEN secret for authentication
- Specify repository slug (timvw/wt) explicitly
- Keep coverage upload only for Go 1.23

Requires CODECOV_TOKEN secret to be configured in repository settings.

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

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

Author: timvw

.github/workflows/ci.yml

@@ -44,8 +44,10 @@ jobs:
 
       - name: Upload coverage to Codecov
         if: matrix.go-version == '1.23'
-        uses: codecov/codecov-action@v4
+        uses: codecov/codecov-action@v5
         with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          slug: timvw/wt
           files: ./coverage.out
           fail_ci_if_error: false