macOS CI: reinstate testing Semigroups on GAP 4.12

It seems to work!

Author: wilfwilson

.github/workflows/standard.yml

@@ -29,12 +29,6 @@ jobs:
           - v4.14
           - v4.13
           - v4.12
-        exclude:
-          # We exclude this combination for the reasons discussed at:
-          # https://github.com/semigroups/Semigroups/pull/1015
-          # https://github.com/gap-system/gap/issues/5640
-          - os: macos
-            gap-version: stable-4.12
 
     steps:
       - uses: actions/checkout@v5