Use a consistent version of spring-javaformat

See gh-19710

buildSrc/build.gradle

 plugins {
 	id 'java-gradle-plugin'
-	id 'io.spring.javaformat' version '0.0.19-SNAPSHOT'
+	id 'io.spring.javaformat' version "$javaFormatVersion"
 	id 'checkstyle'
 }
 
 repositories {
 	mavenCentral()
 	gradlePluginPortal()
+	maven { url 'https://repo.spring.io/snapshot' }
 	maven { url 'https://repo.spring.io/release' }
 }
 
@@ -14,7 +15,7 @@ sourceCompatibility = 1.8
 targetCompatibility = 1.8
 
 dependencies {
-	checkstyle 'io.spring.javaformat:spring-javaformat-checkstyle:0.0.15'
+	checkstyle "io.spring.javaformat:spring-javaformat-checkstyle:$javaFormatVersion"
 	implementation 'com.fasterxml.jackson.core:jackson-databind:2.10.0'
 	implementation 'commons-codec:commons-codec:1.13'
 	implementation 'org.apache.maven:maven-embedder:3.6.2'
@@ -22,7 +23,7 @@ dependencies {
 	implementation 'org.springframework:spring-core:5.2.2.RELEASE'
 	implementation 'org.springframework:spring-web:5.2.2.RELEASE'
 	implementation 'com.google.code.gson:gson:2.8.5'
-	implementation 'io.spring.javaformat:spring-javaformat-gradle-plugin:0.0.15'
+	implementation "io.spring.javaformat:spring-javaformat-gradle-plugin:$javaFormatVersion"
 	testImplementation 'org.assertj:assertj-core:3.11.1'
 	testImplementation 'org.apache.logging.log4j:log4j-core:2.12.1'
 	testImplementation 'org.junit.jupiter:junit-jupiter:5.5.2'

buildSrc/gradle.properties

+javaFormatVersion=0.0.19-SNAPSHOT
\ No newline at end of file