build.gradle: update Bouncy Castle to 1.81

Author: schildbach

core/build.gradle

@@ -10,7 +10,7 @@ version = '0.18-SNAPSHOT'
 
 dependencies {
     api project(':bitcoinj-base')
-    api 'org.bouncycastle:bcprov-jdk15to18:1.80'
+    api 'org.bouncycastle:bcprov-jdk15to18:1.81'
     api 'com.google.guava:guava:33.4.4-android'
     api 'com.google.protobuf:protobuf-javalite:4.30.2'