GH Actions: Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docs.yaml

@@ -14,7 +14,7 @@ jobs:
     if: github.repository == 'PHPMailer/PHPMailer'
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 1
       - name: Build Docs

.github/workflows/scorecards.yml

@@ -25,7 +25,7 @@ jobs:
     
     steps:
       - name: "Checkout code"
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           persist-credentials: false
 

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Set up PHP
         uses: shivammathur/setup-php@v2
@@ -57,7 +57,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install PHP
         uses: shivammathur/setup-php@v2
@@ -110,7 +110,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Set up PHP
         uses: shivammathur/setup-php@v2