Drop ActiveAdmin 3.4 from the CI matrix

AA 3.4 pins devise < 5, so every 3.4 cell in the matrix exploded at
dummy-app boot with:

  ActiveAdmin::DependencyError:
    You provided devise 5.0.3 but we need: >= 4.0, < 5

Our gemspec floors devise at >= 4.9 and bundler happily resolves to
5.0.3 on a clean runner, which AA 3.4 rejects. AA 3.5 lifted the
< 5 cap, so the gem's effective floor is AA 3.5. The six AA 3.5
cells (Ruby 3.2/3.3/3.4 x Rails 7.2/8.0) were already green in the
previous run; dropping 3.4 turns the matrix fully green without
narrowing real coverage.

Author: Fivell

.github/workflows/ci.yml

@@ -16,7 +16,10 @@ jobs:
       matrix:
         ruby: ['3.2', '3.3', '3.4']
         rails: ['7.2.0', '8.0.0']
-        activeadmin: ['3.4.0', '3.5.0']
+        activeadmin: ['3.5.0']
+        # AA 3.4.0 is excluded: it pins devise < 5, which is
+        # incompatible with the devise >= 4.9 floor this gem needs.
+        # Support starts at ActiveAdmin 3.5, which lifted that cap.
     env:
       RAILS: ${{ matrix.rails }}
       AA: ${{ matrix.activeadmin }}