Android: Raise minSdk to 24 (Android 7.0)

Reasons:
- Only 0.000341% of all Android users were on API 21-23 in the last 365 days.
- These devices are old and likely do not run Dolphin well.
- Android 5.x and 6.0 have been without security updates for about 8 years.
- Staying on API 21–23 blocks AndroidX updates, including Lifecycle.

Readme.md

@@ -27,7 +27,7 @@ Please read the [FAQ](https://dolphin-emu.org/docs/faq/) before using Dolphin.
 ### Android
 
 * OS
-    * Android (5.0 Lollipop or higher).
+    * Android (7.0 Nougat or higher).
 * Processor
     * A processor with support for 64-bit applications (either ARMv8 or x86-64).
 * Graphics

Source/Android/app/build.gradle.kts

@@ -43,7 +43,7 @@ android {
 
     defaultConfig {
         applicationId = "org.dolphinemu.dolphinemu"
-        minSdk = 21
+        minSdk = 24
         targetSdk = 36
 
         versionCode = getBuildVersionCode()