ci: Disable julia 1.5

Julia 1.5 on MacOS is giving errors related to GR:

```
GKS: dlopen(/Users/runner/.julia/artifacts/8ad1378af931092f83916905440a69d0af818452/lib/../lib/cairoplugin.so, 1): Library not loaded: @rpath/libtiff.5.dylib
  Referenced from: /Users/runner/.julia/artifacts/8ad1378af931092f83916905440a69d0af818452/lib/cairoplugin.so
  Reason: Incompatible library version: cairoplugin.so requires version 13.0.0 or later, but libtiff.5.dylib provides version 11.0.0
soliton: Error During Test at /Users/runner/.julia/packages/SafeTestsets/A83XK/src/SafeTestsets.jl:25
  Got exception outside of a @test
```

These are only errors in generating plots in the example notebooks, so
don't really matter to the functionality of UltraDark.  However, I don't
actually need to support julia 1.5, so don't run these tests.

Related: JuliaPlots/Plots.jl#3800

.github/workflows/ci.yml

@@ -10,9 +10,9 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.5'
+          # - '1.5'
           - '1.6'
-          # - 'nightly'
+          - 'nightly'
         os:
           - ubuntu-latest
           - macOS-latest