[gradle plugin] Support Gradle 4.10

Gradle 4.10 is bundled with Kotlin 1.60 and Kotlin DSL 1.0-rc1. The new Kotlin DSL isn't binary compatible with the tasks registration used in this plugin.

Updating to Kotlin DSL 1.0-rc1 with no other changes would require users to update to Gradle 4.10.

As a workaround, I've modified the tasks registration being done in OpenApiGeneratorPlugin.kt, so rather than using the Kotlin DSL's invoke, it creates tasks manually against the TasksContainer. This works locally with Gradle 4.7+ for all scenarios in the sample (samples/local-spec). There may be edge cases that I'm unaware of, and we may want to consider defining the minimum supported Gradle version of 4.10 in the next major version of openapi-generator-gradle-plugin if we experience those cases.

See #962 (closed)

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.