Bump actions/checkout from 3 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2

.github/workflows/build_sign_selfhost.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: self-hosted
     if: runner.os == 'macOS' || runner.labels contains 'macos'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Ruby & Fastlane
         run: |
           brew install ruby || true
@@ -29,7 +29,7 @@ jobs:
     runs-on: self-hosted
     if: runner.os == 'Windows' || runner.labels contains 'windows'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Build (MSBuild placeholder)
         shell: powershell
         run: |

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
 
     - name: Setup Flutter
       uses: subosito/flutter-action@v2

.github/workflows/deploy.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2

.github/workflows/signing_selfhost.yml

@@ -10,7 +10,7 @@ jobs:
     name: macOS build & notarize (self-hosted)
     runs-on: [self-hosted, macos]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Ruby & Fastlane
         run: |
           gem install fastlane -NV || true
@@ -27,7 +27,7 @@ jobs:
     name: Windows build & sign (self-hosted)
     runs-on: [self-hosted, windows]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Build via MSBuild (placeholder)
         shell: powershell
         run: |

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2