CI: update to actions/checkout@v6

Author: fingolfin

.github/workflows/CI.yml

@@ -32,7 +32,7 @@ jobs:
           - stable-4.12
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: 'Install additional dependencies'
         run: sudo apt-get install 4ti2 libboost-all-dev # for NormalizInterface
       - uses: gap-actions/setup-gap@v2
@@ -55,7 +55,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: 'Install additional dependencies'
         run: sudo apt-get install 4ti2 libboost-all-dev # for NormalizInterface
       - uses: gap-actions/setup-gap@v2

.github/workflows/release.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: gap-actions/setup-gap@v3
       - uses: gap-actions/build-pkg-docs@v2
         with: