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 23.0.1 to 28.3.1.
- [Release notes](https://github.com/adyen/adyen-java-api-library/releases)
- [Commits](Adyen/adyen-java-api-library@v23.0.1...v28.3.1)

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

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

checkout-example/build.gradle

@@ -19,7 +19,7 @@ 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:26.3.0'
+    implementation 'com.adyen:adyen-java-api-library:28.3.1'
 
     developmentOnly 'org.springframework.boot:spring-boot-devtools'