ci: resolve a couple issues in scheduled updates (#9489)

## Summary

- Switch from running every day to on Mondays and Thursdays
- Update the Github Action, which should hopefully fix an
[error](https://github.com/Esri/calcite-design-system/actions/runs/9310890885/job/25635983212#step:5:566)
where an existing pull request number can't be found
- Make sure the docs update job only commits markdown files
- Delete the branch created by the action when there are no changes to
commit

.github/workflows/schedule-updates.yml

@@ -2,7 +2,7 @@ name: Schedule Updates
 on:
   workflow_dispatch:
   schedule:
-    - cron: "0 0 * * *"
+    - cron: "0 0 * * 1,4"
 jobs:
   component-docs:
     runs-on: ubuntu-latest
@@ -19,10 +19,13 @@ jobs:
           npx --workspace=@esri/calcite-components stencil build --docs
           npm run --workspace=@esri/calcite-components lint:md
       - name: Commit and create pull request
-        uses: peter-evans/create-pull-request@v4
+        uses: peter-evans/create-pull-request@v6
         with:
           base: main
           branch: ci/update-component-docs
+          delete-branch: true
+          add-paths: |
+            **/*.md
           commit-message: "docs: update component READMEs"
           token: ${{ secrets.ADMIN_TOKEN }}
           title: "docs: update component READMEs"
@@ -45,10 +48,11 @@ jobs:
           git config --global user.name "github-actions[bot]"
           npx update-browserslist-db@latest
       - name: Commit and create pull request
-        uses: peter-evans/create-pull-request@v4
+        uses: peter-evans/create-pull-request@v6
         with:
           base: main
           branch: ci/update-browserslist-db
+          delete-branch: true
           commit-message: "build: update browserslist db"
           title: "build: update browserslist db"
           body: "This PR was automatically generated by the update-browserslist-db GitHub action"