Modules for specific databases have to hardcode php versions

[GuySartorelli]

silverstripe/silverstripe-postgresql effectively defines its own matrix which includes hardcoded php versions.
This is because it obviously shouldn't be tested with mysql database.
We might need a new input to disable certain databases to handle this dynamically and avoid the postgres (and possible other db modules in the future e.g. aws) getting out of date.

[GuySartorelli]

Note: While postgres has been removed from the matrix, this work should still be considered in case we do add new database types to the matrix in the future.