ci(tests): remove tmux < 3.2 from test matrix

why: Minimum supported tmux version is now 3.2a, older versions
no longer need CI coverage.
what:
- Remove tmux 2.6, 2.7, 2.8, 3.0a, 3.1b from version matrix
- Keep 3.2a through master for testing

Author: tony

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ['3.14']
-        tmux-version: ['2.6', '2.7', '2.8', '3.0a', '3.1b', '3.2a', '3.3a', '3.4', '3.5', '3.6', 'master']
+        tmux-version: ['3.2a', '3.3a', '3.4', '3.5', '3.6', 'master']
     steps:
       - uses: actions/checkout@v5