android: bump minSdkVersion to 21

Fixes the following error with AGP 8.4:

    A problem occurred configuring project ':webcrypto'.
    > [CXX1110] Platform version 16 is unsupported by this NDK. Please change minSdk to at least 21 to avoid undefined behavior. To suppress this error, add android.ndk.suppressMinSdkVersionError=21 to the project's gradle.properties or set android.experimentalProperties["android.ndk.suppressMinSdkVersionError"]=21 in the Gradle build file.

android/build.gradle

@@ -53,7 +53,7 @@ android {
     }
 
     defaultConfig {
-        minSdkVersion 16
+        minSdkVersion 21
         ndk {
             abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
         }