Start building against Reactor Dysprosium SR19 snapshots

See gh-26768

build.gradle

@@ -28,7 +28,7 @@ configure(allprojects) { project ->
 		imports {
 			mavenBom "com.fasterxml.jackson:jackson-bom:2.10.5"
 			mavenBom "io.netty:netty-bom:4.1.51.Final"
-			mavenBom "io.projectreactor:reactor-bom:Dysprosium-SR17"
+			mavenBom "io.projectreactor:reactor-bom:Dysprosium-BUILD-SNAPSHOT"
 			mavenBom "io.rsocket:rsocket-bom:1.0.4"
 			mavenBom "org.eclipse.jetty:jetty-bom:9.4.31.v20200723"
 			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.3.72"
@@ -279,6 +279,7 @@ configure(allprojects) { project ->
 		repositories {
 			mavenCentral()
 			maven { url "https://repo.spring.io/libs-spring-framework-build" }
+			maven { url "https://repo.spring.io/snapshot" } // reactor
 		}
 	}
 	configurations.all {