ci: drop ubuntu-16.04

GitHub removed Ubuntu Xenial (16.04) runners in Sep, 2021. See the
announcement [1].

[1]: https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/

Author: Totktonada

.github/workflows/test.yml

@@ -19,8 +19,8 @@ jobs:
           - '2.9'
         nightly: [false]
         include:
+        - {runs-on: ubuntu-20.04, tarantool: '1.10', nightly: true}
         - {runs-on: ubuntu-18.04, tarantool: '1.10', nightly: true}
-        - {runs-on: ubuntu-16.04, tarantool: '1.10', nightly: true}
     runs-on: ${{ matrix.runs-on }}
     env:
       TARANTOOL_CACHE_KEY_SUFFIX: -A-${{ github.run_id }}