Bump actions/checkout from 5 to 6 (#21)

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
           - '1.10' # current LTS
           - '1'    # automatically expands to the latest stable 1.x release of Julia
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         persist-credentials: false
     - uses: julia-actions/setup-julia@v2