[Java] Update /for pages

[adinauer]

Description

/for pages could be updated.

• Spring 6 vs 5

• Spring Boot 3 vs 2

• maybe allow toggling Maven / Gradle

• maybe mention source context and point to docs

• check if there's more updates to be made

• https://github.com/getsentry/static-sites/issues/1718

[smeubank]

changes completed others not neeed for marketing now, updated for maven and gradle after the issues in todo