CI - remove ubuntu-18.04 from a test matrix (go-mysql-org#775)

Author: atercattus

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
  strategy:
  matrix:
  go: [ 1.19, 1.18, 1.17, 1.16 ]
- os: [ ubuntu-18.04, ubuntu-20.04 ]
+ os: [ ubuntu-20.04 ]
  name: Tests Go ${{ matrix.go }} # This name is used in main branch protection rules
  runs-on: ${{ matrix.os }}