[kotlin][client] create comment to warn about android api version

[4brunu]

When using kotlin client in Android, we should warn the users about the flag supportAndroidApiLevel25AndBelow to that they make an informed decision

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

[wing328]

@4brunu thanks for the PR, which has been merged. Shall we mark this option as deprecated and remove it in 6.x release in the future?

(e.g. we've dropped JDK7 support in the Java client, server generators in the upcoming v6.0.0 release)

[4brunu]

Hi @wing328, I think we shouldn't do it, because Android API level 25 and below is still used by a lot of devices.
I think we should support at least Android 21 and above.
So I think we should keep this.

[wing328]

👌

Maybe mark it as deprecated in 6.0.0 release (manage expectation low) and schedule the removal in 7.0.0 release a year later. I'll let you decide.

[4brunu]

Honestly I think we should keep it for a few years, because Android adoption moves very slowly.
For you to have an idea, last week I checked analytics in one client of mine, Android API Level 23 that was released in 2015, was the third most used Android version.

[wing328]

Ah ok. Let's keep it then.