Bump actions/upload-artifact from 2 to 3

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 2 to 3.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v2...v3)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/synthetic-validation.yaml

@@ -37,14 +37,14 @@ jobs:
         shell: Rscript {0}
 
       - name: Upload validation figures
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v3
         with:
           name: figures
           retention-days: 5
           path: inst/dev/figs
 
       - name: Upload validation fits
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v3
         with:
           name: fits
           retention-days: 5