Godot 3.x targets minimal API level 18 (Android 4.3). Therefore the error.
Also apps exported with Godot 3.0.x require GLES3 support on the android device.
(This should not be the case for Godot 3.1 projects when set to GLES2. But I didn't test this yet.)
You could try going with Godot 2.x. Or try to build your own android template with a lower SDK version (this might prove difficult if godot actually uses features of the newer API).
https://godotengine.org/qa/38248/choosing-min-and-target-sdks-used-to-compile-android-apk-file
Finally you could put a custom Android version on your S2. I think Cyanogenmod offers a Android 6.0 rom for the S2.