Use latest version of checkout GitHub action

I saw this error in the GitHub Action output:

Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16:
actions/checkout@v2, github/codeql-action/init@v1, github/codeql-action/analyze@v1.
For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

Signed-off-by: Enrico Usai <[email protected]>

Author: Enrico Usai

.github/workflows/changelog_enforcer.yml

@@ -8,7 +8,7 @@ jobs:
   changelog:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@main
     - uses: dangoslen/[email protected]
       with:
         changeLogPath: CHANGELOG.md

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     name: DepCheck
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Dependency Check
         uses: dependency-check/Dependency-Check_Action@main
         with:
@@ -31,7 +31,7 @@ jobs:
           - name: Setup Ruby
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with:
@@ -66,7 +66,7 @@ jobs:
             toxenv: code-linters
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Setup Python
         uses: actions/setup-python@v2
         with:
@@ -79,7 +79,7 @@ jobs:
     name: Shellcheck
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
         with:

.github/workflows/codeql-analysis.yml

@@ -20,7 +20,7 @@ jobs:
         language: [ 'python' ]
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v2
+      uses: actions/checkout@main
     - name: Initialize CodeQL
       uses: github/codeql-action/init@v2
       with:

.github/workflows/system-tests-centos7.yml

@@ -16,7 +16,7 @@ jobs:
         ports:
           - 5000:5000
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Set up Docker Buildx
         id: buildx
         uses: docker/setup-buildx-action@v1

.github/workflows/system-tests-ubuntu.yml

@@ -16,7 +16,7 @@ jobs:
         ports:
           - 5000:5000
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Set up Docker Buildx
         id: buildx
         uses: docker/setup-buildx-action@v1