Upgrade tests projects target framework to net472,netcoreapp3.1

[Ste1io]

netcoreapp2.1 is out of support; this causes both warning NETSDK1138 when building the test projects, and has turned out to be non-starter for adding official Oracle support (#699) which is well underway and progressing otherwise-smoothly thanks to @Curlack's work in #702.

Oracle.ManagedDataAccess is the framework version (hence no issues with net472 target). Oracle.ManagedDataAccess.Core is the "modern" package for core apps (click links for compatible versions on nuget).

Unfortunately, the lowest compatable dotnetcore version for the "Oracle.ManagedDataAccess.Core" package is dotnetcore3.1.

Originally posted by @Ste1io in #702 (comment)