ci: remove testing on ubuntu-18.04

It has been deprecated sometime ago:
actions/runner-images#6002

Author: grafin

.github/workflows/test.yml

@@ -30,7 +30,6 @@ jobs:
         # so just use it.
         - {runs-on: ubuntu-22.04, tarantool: '2.10', nightly: false}
         - {runs-on: ubuntu-20.04, tarantool: '1.10', nightly: true}
-        - {runs-on: ubuntu-18.04, tarantool: '1.10', nightly: true}
     runs-on: ${{ matrix.runs-on }}
     env:
       TARANTOOL_CACHE_KEY_SUFFIX: -A-${{ github.run_id }}