Update Spring example with jakarta validation dependency

[Philzen]

Spring Boot 2.2 switched many javax.*-packages to jakarta, as per

https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.2-Release-Notes#jakarta-ee-dependencies

Changing the example POM to align with this change to avoid introducing two conflicting versions of the same namespace into a project.

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[Philzen]

BTW – i realized https://github.com/OpenAPITools/openapi-generator/blob/v7.0.1/modules/openapi-generator-maven-plugin/examples/java-client.xml uses a newer version for swagger-annotations, namely 1.6.3 – while the spring example still uses 1.5.8.

1. Should this be harmonized as well?
2. What about 1.6.11 – which seems the most recent one from the 1.x branch – has that been tested for compatibility?

Oh dear, looks like they both need updates, pls see discussion for clarification in #16701

[Philzen]

Could not test the springfox-swagger update as – so far – our projects don't need them, however from reading their documentation the 3.x version seems to be the way to go and is compatible with the Spring Boot Version in the examples: https://github.com/springfox/springfox#migrating-from-existing-2x-version

[wing328]

I've merged #16720. Do you still need this?

happy to reopen this PR if needed

[Philzen]

@wing328 Thanks! #16720 clarified a lot for me b/c i was already wondering whether

         <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>

wouldn't be the better choice here – as it will transitively handle exactly the jakarta. vs. javax. confusion.