ci: add Tarantool 2.8 to testing

Tarantool 2.8 has been released in Aug 2021 [1], I don't see any reasons
why we should skip testing for it.

1. https://www.tarantool.io/en/doc/latest/release/2021-08-releases/

Author: ligurio

.github/workflows/test_on_push.yaml

@@ -18,7 +18,7 @@ jobs:
         include:
           - tarantool-version: "2.7"
             remove-merger: true
-          - tarantool-version: "2.7"
+          - tarantool-version: "2.8"
             coveralls: true
       fail-fast: false
     runs-on: [ubuntu-latest]