ci: bump actions/checkout to v5 in sync workflows

Silences the Node 20 deprecation warning on the auto-sync runs.

Author: gburd

.github/workflows/sync-upstream-manual.yml

@@ -18,7 +18,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
         with:
           fetch-depth: 0
           # PAT with repo + workflow scope. The default GITHUB_TOKEN cannot push

.github/workflows/sync-upstream.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
         with:
           fetch-depth: 0
           # PAT with repo + workflow scope. The default GITHUB_TOKEN cannot push