docs+ci: bump actions/checkout to v7 (README, example workflow, tag mover)

v7's only behavior change blocks fork-PR checkout under
pull_request_target/workflow_run triggers (actions/checkout#2454); nothing
here uses those. Coordinated with pgup-ai/jbot-review#90 and the landing
site so every documented snippet pins the same major.

Author: jingbof

.github/workflows/move-v0-tag.yml

@@ -12,7 +12,7 @@ jobs:
   move-v0:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
         with:
           fetch-depth: 0
           fetch-tags: true

README.md

@@ -77,7 +77,7 @@ jobs:
     if: github.event.pull_request.draft == false
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
         with:
           fetch-depth: 0
       - uses: pgup-ai/jbot-review-action@v0 # latest v0.x.y

examples/jbot-review.yml

@@ -137,7 +137,7 @@ jobs:
       cancel-in-progress: true
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
         with:
           fetch-depth: 0
           # issue_comment and workflow_dispatch run on the default branch, so