ci: align stray actions/checkout pins to v6.0.2 (#702)

Two jobs (ci.yml posthog-sync-plan, publish-middleware-python) were on the
v4.3.1 checkout SHA while the rest of the repo uses v6.0.2. Align them.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: blove

.github/workflows/ci.yml

@@ -969,7 +969,7 @@ jobs:
     if: github.event_name == 'push' || needs.ci-scope.outputs.posthog == 'true'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
       - name: Detect PostHog-relevant changes

.github/workflows/publish-middleware-python.yml

@@ -53,7 +53,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install uv
         uses: astral-sh/setup-uv@d4b2f3b6ecc6e67c4457f6d3e41ec42d3d0fcb86 # v5.4.2