Remove Explicit Support for Play 2.3 and Gradle pre-6

Per @JLLeitschuh 's request on #136 .

Removal of support comprises 3 aspects:

1. Updating docs to reflect new supported range.
2. Updating Test Coverage to no longer include Play 2.3 and Gradle 5.x versions
3. Updating Plugin code to no longer include Play 2.3, and issue a warning if this is used, as follows:

- PlayApplicationPlugin.apply: fail fast and explicitly if a specific version is declared, which is unsupported
- PlayDistributionPlugin.getMainClass: no longer suggest the NettyServer
- RoutesCompilerAdapterV23X: remove
- RoutesCompilerFactory remove 2.3 code
- PlayRunAdapterV23X remove
- PlayApplicationRunnerFactory remove 2.3 code
- PlayTwirlAdapterV23X remove
- TwirlCompilerFactory remove 2.3 code