gradle: update kotlin to 2.1.20

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 agp = "8.9.2"
-kotlin = "1.9.0"
+kotlin = "2.1.20"
 
 [libraries]
 androidx-activity-ktx = "androidx.activity:activity-ktx:1.8.0"

ui/src/main/java/com/wireguard/android/widget/SlashDrawable.kt

@@ -69,6 +69,7 @@ class SlashDrawable(private val mDrawable: Drawable) : Drawable() {
 
     override fun getIntrinsicWidth() = mDrawable.intrinsicWidth
 
+    @Deprecated("Deprecated in API level 29")
     override fun getOpacity() = PixelFormat.OPAQUE
 
     override fun onBoundsChange(bounds: Rect) {