libretro: increase ANDROID_API to 21 for android builds

[cscd98]

libretro#63

We now use NDK 29 on the libretro buildbot which does not support API level 16.

Hopefully fixes this:

Preparing ARMeabi-v7a toolchain **** Building zlib **** CMake Error at /opt/android-sdk-linux/cmake/3.30.3/share/cmake-3.30/Modules/Platform/Android-Determine.cmake:503 (message): Android: The API level 16 is not supported by the NDK. Choose one in the range of [21, 35].

There might be further issues after this.

[carstene1ns]

Jenkins: test this please.

We use API 21 for our buildscripts/android port as well¹, so should likely work without further problems.

Footnotes

1. https://github.com/EasyRPG/buildscripts/blob/84cf2f449a0d1c09f17d2bbf3d28f4f573aa718a/android/2_build_toolchain.sh#L48 ↩