Bump com.adyen:adyen-java-api-library in /checkout-example

Bumps [com.adyen:adyen-java-api-library](https://github.com/adyen/adyen-java-api-library) from 21.0.0 to 21.4.0.
- [Release notes](https://github.com/adyen/adyen-java-api-library/releases)
- [Commits](Adyen/adyen-java-api-library@v21.0.0...v21.4.0)

---
updated-dependencies:
- dependency-name: com.adyen:adyen-java-api-library
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

checkout-example/build.gradle

@@ -18,7 +18,7 @@ dependencies {
 	implementation 'org.springframework.boot:spring-boot-starter-thymeleaf'
 	implementation 'org.springframework.boot:spring-boot-starter-web'
 	implementation 'nz.net.ultraq.thymeleaf:thymeleaf-layout-dialect'
-	implementation 'com.adyen:adyen-java-api-library:21.0.0'
+	implementation 'com.adyen:adyen-java-api-library:21.4.0'
 	developmentOnly 'org.springframework.boot:spring-boot-devtools'
 	testImplementation('org.springframework.boot:spring-boot-starter-test') {
 		exclude group: 'org.junit.vintage', module: 'junit-vintage-engine'