Use current checkout action in workflows

Author: ripred

.github/workflows/arduino-lint.yml

@@ -8,7 +8,7 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: arduino/arduino-lint-action@v2
         with:
           library-manager: update

.github/workflows/arduino_test_runner.yml

@@ -12,7 +12,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       # BUGBUG
       # DO-NOTHING FOR NOW - TIRED OF STUPID WORKFLOW FAILING

.github/workflows/jsoncheck.yml

@@ -12,7 +12,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: json-syntax-check
         uses: limitusus/json-syntax-check@v1
         with: