Update Webapp Template to Spring Boot version 4

Current Version of spring boot is `3.5.5`. 
Projects at WeAreFrank have/are moving to version `4.x.x` (4.0.3 is latest as of writing this)

Template library should be updated to match this to prevent having to migrate projects unnecessarily.

Could also be used as an opportunity to move from `Java 21 LTS` to `Java 25 LTS` and to move from `Angular v20` to `Angular v21`

