Add so-simulator integration test

Add so-simulator integration test

- define integration test to assert that the so-simulator
  has a working application context
- change spring.boot.version to springboot.version in so-simulator module to override the
  global property of the same name. This ensures that all spring boot deps of this module
  are in version 2.0.9 (before, some were in 2.4.12 which lead to incompatibilities)
- add dependency management for spring cloud in so-simulator module to make sure that it
  is in a spring boot 2.0.9-compatible version
- override javassist version (3.22.0-GA -> 3.25.0-GA) to have a java 11 compatible version
- downgrade h2 dependency (2.1.214 -> 1.4.200) to be compatible with the citrus-simulator
  dependencies
- update logback (1.2.7 -> 1.2.11)

Issue-ID: SO-4200
Change-Id: I2e58cc0c59193a0a1268958a398699e577f25b1d
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>