SDL: Use the android-21 SDK as documented in README-android.md

SDL Commits
1 
From ac287025042231083b1283ad7f671d2a133d56fe Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Thu, 20 Mar 2025 10:47:43 -0700
Subject: [PATCH] Use the android-21 SDK as documented in README-android.md

---
 android-project/app/build.gradle | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/android-project/app/build.gradle b/android-project/app/build.gradle
index 8946de66778aa..42221bfadcbcc 100644
--- a/android-project/app/build.gradle
+++ b/android-project/app/build.gradle
@@ -14,12 +14,12 @@ android {
         versionName "1.0"
         externalNativeBuild {
             ndkBuild {
-                arguments "APP_PLATFORM=android-19"
+                arguments "APP_PLATFORM=android-21"
                 // abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
                 abiFilters 'arm64-v8a'
             }
             cmake {
-                arguments "-DANDROID_PLATFORM=android-19", "-DANDROID_STL=c++_static"
+                arguments "-DANDROID_PLATFORM=android-21", "-DANDROID_STL=c++_static"
                 // abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
                 abiFilters 'arm64-v8a'
             }