fix: Target Fedora 40 for official images, revert 39 to ci-test

Official images aren't quite ready yet

.github/workflows/build.yml

@@ -68,7 +68,7 @@ jobs:
       - name: Matrix Variables
         shell: bash
         run: |
-          if [[ "${{ matrix.major_version }}" -ge "39" ]] && \
+          if [[ "${{ matrix.major_version }}" -ge "40" ]] && \
             grep "${{ matrix.image_name }}" <<< "silverblue, kinoite, sericea"; then
               echo "SOURCE_ORG=fedora" >> $GITHUB_ENV
               echo "SOURCE_IMAGE=fedora-${{ matrix.image_name }}" >> $GITHUB_ENV