use latest Julia v1 for docs (#67)

Author: ranocha

.github/workflows/Documenter.yml

@@ -36,7 +36,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1
         with:
-          version: '1.6'
+          version: '1'
           show-versioninfo: true
       - uses: julia-actions/cache@v1
       - uses: julia-actions/julia-buildpkg@v1