gha: update test-matrix: remove docker 26.x, add 25.x

- Mirantis Container Runtime (MCR) 23.0 reached EOL, and the next LTS
  version of MCR is 25.x
- Docker 26.x reached EOL and is no longer maintained

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

.github/workflows/ci.yml

@@ -156,7 +156,7 @@ jobs:
           - plugin
           - standalone
         engine:
-          - 26
+          - 25
           - 27
           - 28
     steps: