Use bitcoin-kmp 0.21.0 (#732)

Author: sstone

gradle/libs.versions.toml

@@ -4,8 +4,8 @@ kotlinx-coroutines = "1.7.3"
 kotlinx-datetime = "0.6.0"
 kotlinx-serialization = "1.6.2"
 ktor = "2.3.7"
-bitcoinkmp = "0.20.0" # when upgrading bitcoin-kmp, keep secpjnijvm in sync!
-secpjnijvm = "0.15.0"
+bitcoinkmp = "0.21.0" # when upgrading bitcoin-kmp, keep secpjnijvm in sync!
+secpjnijvm = "0.16.0"
 kermit = "2.0.2"
 slf4j = "1.7.36"