Consider switching Logback's file logging charset to UTF-8

Right now, Logback uses `Charset.defaultCharset()` for file logging. Log4j2 uses UTF-8. I think we should consider using UTF-8 for Logback, too. `Charset.defaultCharset()` is platform dependent, which could lead to confusing bugs. The Java team (with JEP 400) recognized this is a problem and fixed that in Java 18.

Read https://github.com/spring-projects/spring-boot/pull/44353 for more background.