chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/charm-check-libs.yaml

@@ -20,7 +20,7 @@ jobs:
       contents: read
       pull-requests: write # required to label PR based on results
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Check libraries

.github/workflows/lint.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Run Ruff

.github/workflows/promote-charm.yaml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Promote

.github/workflows/publish-charm.yaml

@@ -19,7 +19,7 @@ jobs:
       packages: write
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Upload charm to charmhub

.github/workflows/validate_workflows.yaml

@@ -24,7 +24,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Run zizmor