CI.yaml: Stop testing on PHP 7.2/7.3

Author: reedy

.github/workflows/CI.yaml

@@ -10,12 +10,6 @@ jobs:
  fail-fast: false
  matrix:
  include:
- # Includes php7.1-8.0, composer 2, php-xdebug, and more
- # https://github.com/actions/virtual-environments/blob/ubuntu18/20210318.0/images/linux/Ubuntu1804-README.md#php
- - os: ubuntu-18.04
- php: "7.2"
- - os: ubuntu-18.04
- php: "7.3"
  # Includes php7.4-8.x, composer 2, php-xdebug, and more
  # https://github.com/actions/virtual-environments/blob/ubuntu20/20210318.0/images/linux/Ubuntu2004-README.md#php
  - os: ubuntu-20.04