Change upload artifact github action version

Version 2 of this github action has been deprecated, we shall bump the
version used in the CI.

ChangeLog:

	* .github/workflows/bootstrap.yml: Bump version from 2 to 4.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

.github/workflows/bootstrap.yml

@@ -49,7 +49,7 @@ jobs:
            cd gccrs-build; \
            make check-rust
     - name: Archive check-rust results
-      uses: actions/upload-artifact@v2
+      uses: actions/upload-artifact@v4
       with:
         name: check-rust-logs
         path: |