Bump actions/checkout from 4 to 6 (qmk#25829)

Author: dependabot[bot]

.github/workflows/bootstrap_testing.yml

@@ -105,7 +105,7 @@ jobs:
           echo 'Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"' >> /etc/sudoers
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 1
           submodules: recursive
@@ -174,7 +174,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 1
           submodules: recursive
@@ -224,7 +224,7 @@ jobs:
             git:
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 1
           submodules: recursive