Context:
As Camunda 7.15 gets released at the end of quarter 1 Optimize needs to officially support this new minor release while 7.12 support will be dropped

AT:

• Camunda Platform test version is raised to 7.15
  • 7.15 is added to supported versions in the engine version checker
• Camunda Platform model version is raised to 7.15
• engine integration tests are extended with a 7.15 stage
• Supported Environment in docs are updated accordingly
• drop of support for 7.12 support
• removal of 7.12 engine integration test stage
• drop of support for 7.12 is listed in the update notes to 3.4.0