Gradle legacy plugin application instead of plugins DSL

[fondesa]

Since we just started, we are applying plugin using the new plugins DSL.
I recommend switching to the legacy plugin application because it's more flexible and doesn't require any workaround to solve issues like this one: gradle/gradle#20084

For example, without the workarounds suggested in the issue linked above, we can't depend on the kotlin plugin in build-tools having the kotlin plugin applied with new convention also in other projects.

[fondesa]

@4face-studi0 @mcatta

[fardavide]

All good for me.
I'd suggest having a minimal PR to make the changes clear to everyone

[fondesa]

PR with the proposal: #16