Bump actions/checkout from 5 to 6

.github/workflows/deploy.yaml

@@ -13,7 +13,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0  # fetch the entire repo history, required to guarantee setuptools_scm will pick up tags
       - uses: actions/setup-python@v6

.github/workflows/docs.yaml

@@ -6,7 +6,7 @@ jobs:
     name: Run sphinx-build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0  # fetch the entire repo history, required to guarantee versioneer will pick up the tags
       - uses: astral-sh/setup-uv@v7

.github/workflows/pytest.yaml

@@ -11,7 +11,7 @@ jobs:
         python-version: ["3.11", "3.12", "3.13"]
         pycalphad_develop_version: [true, false]
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0  # fetch the entire repo history, required to guarantee versioneer will pick up the tags
     - uses: astral-sh/setup-uv@v7