build(deps): bump actions/upload-artifact from 3 to 4

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v3...v4)

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

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

Author: dependabot[bot]

.github/workflows/ex-rtd.yml

@@ -157,7 +157,7 @@ jobs:
         run: python ci_create_examples_rst.py
 
       - name: Upload artifacts
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: rtd-files-for-${{ needs.set_options.outputs.sha }}
           path: |

.github/workflows/ex-workflow.yml

@@ -107,7 +107,7 @@ jobs:
         shell: python
 
       - name: Upload zipfile
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: ${{ env.DISTNAME }}.zip
           path: modflow6-examples/${{ env.DISTNAME }}.zip
@@ -201,7 +201,7 @@ jobs:
 
       - name: Upload PDF document
         if: matrix.python == '3.9'
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: ${{ env.DISTNAME}}.pdf
           path: modflow6-examples/${{ env.DISTNAME }}.pdf