Update Tarantool versions in test matrix

Tarantool 2.8.1 [1] and Tarantool 2.9 are available [2], let's include
it to test matrix. Test
simple_operations[memtx].test_intermediate_nullable_fields_update has
failed on 2.8 [3] and to crash looks similar to a bug described in [4].
Unfortunately luatest has lack of XFail support [5], so broken test is
skipped on Tarantool versions greater than 2.8.

Tarantool 2.4-6 has reached EOL, so these version has been removed from
test matrix.

1. https://github.com/tarantool/tarantool/releases/tag/2.8.1
2. https://github.com/tarantool/tarantool/releases/tag/2.9.0
3. https://github.com/tarantool/crud/pull/195/checks?check_run_id=3320469716
4. tarantool/tarantool#6069
5. tarantool/luatest#158

Author: ligurio

.github/workflows/test_on_push.yaml

@@ -11,7 +11,7 @@ jobs:
       github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository
     strategy:
       matrix:
-        tarantool-version: ["1.10", "2.2", "2.3", "2.4", "2.5", "2.6", "2.7"]
+        tarantool-version: ["1.10", "2.2", "2.3", "2.7", "2.8", "2.9"]
         remove-merger: [false]
         include:
           - tarantool-version: "2.7"

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Changed
 
+* Tarantool versions 2.{4,5,6} have reached EOL and removed from regression
+  testing.
 * Names of errors generated by CRUD operations have been unified.
 
 ### Added

test/integration/simple_operations_test.lua

@@ -394,6 +394,8 @@ pgroup:add('test_upsert', function(g)
 end)
 
 pgroup:add('test_intermediate_nullable_fields_update', function(g)
+    -- Blocked by https://github.com/tarantool/tarantool/issues/6069
+    t.skip_if(_TARANTOOL > "2.8")
     local result, err = g.cluster.main_server.net_box:call(
         'crud.insert', {'developers', {1, box.NULL}})
     t.assert_equals(err, nil)