[kotlin] back ticks escaping of keywords and more

[grzegorz-moto]

This fixes #11148 where Kotlin keywords are badly escaped
This also fixes #8687 by removing () from base class name
Based on info from #4537 and #8687

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[wing328]

cc @jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[wing328]

@grzegorz-moto thanks for the PR. I think you will need to remove EOF (which is likely added automatically by your text editor)

Same for https://github.com/OpenAPITools/openapi-generator/pull/11166/files#diff-f2fe24f5af2d6f356946c5a9aeff0eedc692f07aa12d6dd8658e4c1c9bff9c38R3

[grzegorz-moto]

Done. Thanks

[4brunu]

Looks good to me 👍