Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/pr-close-signal.yaml

@@ -16,7 +16,7 @@ jobs:
           mkdir -p ./pr
           printf ${{ github.event.number }} > ./pr/NUM
       - name: Upload Diff
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: pr
           path: ./pr

.github/workflows/pr-receive.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: "Upload PR number"
         id: upload
         if: ${{ always() }}
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: pr
           path: ${{ github.workspace }}/NR
@@ -107,21 +107,21 @@ jobs:
         shell: Rscript {0}
 
       - name: "Upload PR"
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: pr
           path: ${{ env.PR }}
           overwrite: true
 
       - name: "Upload Diff"
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: diff
           path: ${{ env.CHIVE }}
           retention-days: 1
 
       - name: "Upload Build"
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: built
           path: ${{ env.MD }}