[go-migration] Fix replacing of launcher class for spring-boot apps, refactor spring-boot container

[kiril-keranov]

• Fix replacing of the specified Spring-boot launcher class in the corresponding application manifest to always use JarLauncher for spring-boot exploded jars. For example if another launcher is specified in the app manifest like Main-Class: org.springframework.boot.loader.launch.PropertiesLauncher it will be replaced with org.springframework.boot.loader.launch.JarLauncher for spring-boot 3.x apps or org.springframework.boot.loader.JarLauncher for spring-boot 2.x apps when building the start command for this application. Seems this is due to some AI generation issue as the current buildpack comprises no such logic.
• Also some code refactoring is applied in the spring-boot container to prevent code duplication.
• Fix failing tests with regard to -Xbootclasspath/a: removal with previous PR.

[ramonskie]

lgtm