Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build_tests.yml

@@ -23,7 +23,7 @@ jobs:
   build_tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: ${{ github.head_ref }}
       - name: Setup Python

.github/workflows/dev2master.yml

@@ -8,7 +8,7 @@ jobs:
   build_and_publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # otherwise, there would be errors pushing refs to the destination repository.
           ref: dev

.github/workflows/license_tests.yml

@@ -15,7 +15,7 @@ jobs:
   license_tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: ${{ github.head_ref }}
       - name: Setup Python

.github/workflows/publish_alpha.yml

@@ -22,7 +22,7 @@ jobs:
   build_and_publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: dev
           fetch-depth: 0 # otherwise, there would be errors pushing refs to the destination repository.

.github/workflows/publish_build.yml

@@ -8,7 +8,7 @@ jobs:
   build_and_publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: dev
           fetch-depth: 0 # otherwise, there would be errors pushing refs to the destination repository.

.github/workflows/publish_major.yml

@@ -8,7 +8,7 @@ jobs:
   build_and_publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: dev
           fetch-depth: 0 # otherwise, there would be errors pushing refs to the destination repository.

.github/workflows/publish_minor.yml

@@ -8,7 +8,7 @@ jobs:
   build_and_publish:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: dev
           fetch-depth: 0 # otherwise, there would be errors pushing refs to the destination repository.

.github/workflows/unit_tests.yml

@@ -38,7 +38,7 @@ jobs:
         python-version: [ 3.7, 3.8, 3.9, "3.10" ]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - name: Set up python ${{ matrix.python-version }}
         uses: actions/setup-python@v2
         with: