Remove the kotlin-dsl plugin

[martinbonnin]

Closes #108

It turns out the kotlin-dsl plugin embeds code that's using Gradle 6.1 classes while we shouldn't really need this. Remove it and use the plain java-gradle-plugin instead.

Also add some checks and tests about Gradle version.

[martinbonnin]

I bumped from 5.0 to 5.4.1 because the testProject fails on 5.0 due to:

Build file '/home/martin/git/swagger-gradle-codegen/gradle-plugin/plugin/junit7355662627637669365/project/build.gradle.kts' line: 9

* What went wrong:
Script compilation errors:

  Line 09:       content {
                 ^ Unresolved reference: content

  Line 10:         excludeGroup("com.yelp.codegen")
                   ^ Unresolved reference: excludeGroup

I think it should be safe to assume most people are using gradle >= 5.4.1 but we can certainly remove that limitation if really needed.

[macisamuele]

@martinbonnin I would personally prefer to use kotlin-dsl as far as possile.
I did open #111 to deal with the problem mentioned in #108 .

The approach proposed on the other PR has one "major" downside: the project itself cannot benefit from the latest gradle features. But I don't think it is going to be a major blocker.

[cortinico]

@martinbonnin I would personally prefer to use kotlin-dsl as far as possile.

+1

[martinbonnin]

@macisamuele no problem if you want to keep it but as you said, it's bringing a downside and I don't see the upside. The amount of code is almost similar and it might bring things like HasImplicitReceiver to allow to use this instead of it but this almost sounds like a downside too. For someone used to Kotlin, this transformation looks a bit magic.
Or are there more advantages ?

[cortinico]

Or are there more advantages ?

kotlin-dsl plugin allows us to use the precompiled plugins:
https://docs.gradle.org/current/userguide/kotlin_dsl.html#kotdsl:precompiled_plugins
(They actually added a note that covers exactly our scenario)

That is convenient as it simplifies our Gradle setup. I'll try to stick to it as long as it's possible. Although, I don't have a really strong opinion on this, we could even drop if you have a strong argument @martinbonnin 👍

[cortinico]

Following up the discussions we had on Slack here

We decided for replacing kotlin-dsl plugin with java-gradle-plugin for the reasons aforementioned by @martinbonnin.
I'm merging this and will prepare an upcoming release soon 👍