ci: Exclude macos tests below 27.x

See purcell/setup-emacs#48.

Author: jcs090218

.github/workflows/main.yml

@@ -32,6 +32,11 @@ jobs:
           - os: macos-latest
             emacs-version: snapshot
             experimental: true
+        exclude:
+          - os: macos-latest
+            emacs-version: 26.3
+          - os: macos-latest
+            emacs-version: 27.2
 
     steps:
     - uses: jcs090218/setup-emacs@master