Clean the Logback instrumentation of the OTel starter

[jeanbisutti]

Related to #11610

The new implementation does not change the behavior seen by te user (even if the OpenTelemetry appender was added even if it was not necessary) because the installation of OpenTelemetry into the appender depends on the otel.sdk.disabled and otel.instrumentation.logback-appender.enabled properties:

opentelemetry-java-instrumentation/instrumentation/spring/spring-boot-autoconfigure/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/instrumentation/logging/OpenTelemetryAppenderAutoConfiguration.java

Line 35 in de11929

@ConditionalOnEnabledInstrumentation(module = "logback-appender")

opentelemetry-java-instrumentation/instrumentation/spring/spring-boot-autoconfigure/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/internal/ConditionalOnEnabledInstrumentation.java

Line 23 in de11929

@Conditional({SdkEnabled.class, InstrumentationPropertyEnabled.class})

The new tests have the same behavior with the previous implementation and the new one.