Clarify Java runtime prerequisites

[kwin]

Seems that at least the most recent version 3.3.1 requires Java 17+ at runtime. Would be good to clarify in the readme or documentation the minimum Java requirements per version.

[kwin]

Currently only the build prerequisites are outlined at https://github.com/spring-projects/spring-ldap?tab=readme-ov-file#prerequisites.

[jzheaux]

Thanks for the suggestion, @kwin. Sorry if this was a surprise to you. I agree that it would be helpful to add a Prerequisites page to the reference manual so others aren't surprised by this.

We could follow Spring Security's Prerequisite page for a model.

Also since Spring LDAP follows Spring Framework's baseline, it will likely help to include that rule of thumb on the same page.

Are you able to add a Prerequisites page in a PR?