Set spring-security.version to 5.5.0-M1 in sample

samples/boot/oauth2-integration/authorizationserver/spring-security-samples-boot-oauth2-integrated-authorizationserver.gradle

@@ -1,7 +1,7 @@
 apply plugin: 'io.spring.convention.spring-sample-boot'
 
 // TODO Remove once Spring Boot upgrades to Spring Security 5.5
-ext['spring-security.version'] = '5.5.+'
+ext['spring-security.version'] = '5.5.0-M1'
 
 dependencies {
 	compile 'org.springframework.boot:spring-boot-starter-web'