Pin image to specific version so that we don't get surprise failures
down the line.
Use common action to configure the default PHP version (the matrix
versions only affect test running).
Update PHP versions to supported versions in this image.
In a subsequent patch #120, we bump eslint-config-wikimedia to
0.17.0, which also requires bumping eslint to 7.9.0 which drops
support for Node.js 8.x
Bug: T266462
Two workflows have been added,
* PHP build - Runs tests on version 7.2, 7.3, also runs phpcs
* Node.js build - Runs tests on version 8.x, 10.x, 12.x, also
runs ESLint
Bug: T218639
