Upgrade to Reactor Bismuth-M4

Closes gh-10121

Author: snicoll

spring-boot-dependencies/pom.xml

@@ -143,7 +143,7 @@
 		<postgresql.version>42.1.4</postgresql.version>
 		<quartz.version>2.3.0</quartz.version>
 		<querydsl.version>4.1.4</querydsl.version>
-		<reactor-bom.version>Bismuth-BUILD-SNAPSHOT</reactor-bom.version>
+		<reactor-bom.version>Bismuth-M4</reactor-bom.version>
 		<reactive-streams.version>1.0.1</reactive-streams.version>
 		<rxjava.version>1.3.1</rxjava.version>
 		<rxjava-adapter.version>1.2.1</rxjava-adapter.version>