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/main/java/eu/kanade/tachiyomi/ui/base/activity/BaseThemedActivity.kt

@@ -32,10 +32,6 @@ abstract class BaseThemedActivity : AppCompatActivity() {
                         R.style.Theme_Tachiyomi_Light_Api27
                     }
                     // Light status bar + fallback gray navigation bar
-                    Build.VERSION.SDK_INT >= Build.VERSION_CODES.M -> {
-                        R.style.Theme_Tachiyomi_Light_Api23
-                    }
-                    // Fallback gray status + navigation bar
                     else -> {
                         R.style.Theme_Tachiyomi_Light
                     }

app/src/main/java/eu/kanade/tachiyomi/ui/base/controller/ConductorExtensions.kt

@@ -2,7 +2,6 @@ package eu.kanade.tachiyomi.ui.base.controller
 
 import android.content.Intent
 import android.content.pm.PackageManager.PERMISSION_GRANTED
-import android.os.Build
 import androidx.core.content.ContextCompat
 import androidx.core.net.toUri
 import com.bluelinelabs.conductor.Controller
@@ -21,11 +20,9 @@ fun Router.popControllerWithTag(tag: String): Boolean {
 
 fun Controller.requestPermissionsSafe(permissions: Array<String>, requestCode: Int) {
     val activity = activity ?: return
-    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
-        permissions.forEach { permission ->
-            if (ContextCompat.checkSelfPermission(activity, permission) != PERMISSION_GRANTED) {
-                requestPermissions(arrayOf(permission), requestCode)
-            }
+    permissions.forEach { permission ->
+        if (ContextCompat.checkSelfPermission(activity, permission) != PERMISSION_GRANTED) {
+            requestPermissions(arrayOf(permission), requestCode)
         }
     }
 }