bump reactor-core to 3.4.x version and replace deprecated API usage (#2458)

diff --git a/gradle.properties b/gradle.properties
index b168d15..aeab7cf 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -15,7 +15,7 @@
 lincheck_version=2.10
 dokka_version=0.9.16-rdev-2-mpp-hacks
 byte_buddy_version=1.10.9
-reactor_version=3.2.5.RELEASE
+reactor_version=3.4.1
 reactive_streams_version=1.0.2
 rxjava2_version=2.2.8
 rxjava3_version=3.0.2