Un-bump AGP to 8.1.2 (facebook#39958)

Summary:

I'm reverting the change of AGP from 8.2 beta to 8.1 as we don't need 8.2
Using 8.2 beta forces us to use Android Studio beta, which is actually not necessary.

Changelog:
[Android] [Changed] - Bump AGP to 8.1.2

Reviewed By: yungsters

Differential Revision: D50016572

packages/react-native/gradle/libs.versions.toml

@@ -5,7 +5,7 @@ targetSdk = "34"
 compileSdk = "34"
 buildTools = "34.0.0"
 # Dependencies versions
-agp = "8.2.0-beta06"
+agp = "8.1.2"
 androidx-annotation = "1.6.0"
 androidx-appcompat = "1.6.1"
 androidx-autofill = "1.1.0"