Update `SystemEnvironmentPropertyMapper` to generate both mappings in both the original case and upper case. This restores the behavior of Spring Boot 3.4 where the system environment could contain lowercase names. Fixes gh-45741
Update `SystemEnvironmentPropertyMapper` to generate both mappings in both the original case and upper case. This restores the behavior of Spring Boot 3.4 where the system environment could contain lowercase names. Fixes gh-45741