Bump Julia version to 1.8 in Documentation.yaml to fix the CI

Author: ferrolho

.github/workflows/Documentation.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1
         with:
-          version: '1.6'
+          version: '1.8'
       - name: Install dependencies
         run: julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'
       - name: Build and deploy