Drop support for Android 5.x

It's 5-6 years old, and only accounts for 2% of users in the Firebase analytics.

app/src/test/java/eu/kanade/tachiyomi/data/database/CategoryTest.kt

@@ -13,7 +13,7 @@ import org.junit.runner.RunWith
 import org.robolectric.RuntimeEnvironment
 import org.robolectric.annotation.Config
 
-@Config(constants = BuildConfig::class, sdk = [Build.VERSION_CODES.LOLLIPOP])
+@Config(constants = BuildConfig::class, sdk = [Build.VERSION_CODES.M])
 @RunWith(CustomRobolectricGradleTestRunner::class)
 class CategoryTest {