(doc) Update log4j-spring-boot spring-boot min version

[SebHeuze]

Since log4j-spring-boot 2.18.0 the required Spring Boot version is now 2.4.0 since it use LoggingSystemFactory which is a spring boot 2.4.0 class
https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/logging/LoggingSystemFactory.html

[ppkarwasz]

Hi @SebHeuze,

The story is a little bit more complicated than that.

Before Spring Boot 2.4.0 the only way to select a Spring Boot LoggingSystem was to set a system property:

org.springframework.boot.logging.LoggingSystem=org.apache.logging.log4j.spring.boot.Log4j2SpringBootLoggingSystem

This method is still valid with all the Spring Boot versions mentioned in the docs.

Log4j2 2.17.2 or earlier used a log4j2.system.properties file (cf. System Properties) embedded in the log4j-spring-boot JAR file to set this property. This method effectively prevents the usage of any other LoggingSystem (including the original Spring Boot's Log4j2LoggingSystem) and was removed in #777.

Since Spring Boot 2.4.0 a new mechanism to register a LoggingSystem is available: spring.factories (cf. SpringFactoriesLoader. This is the mechanism adopted by Log4j 2.18.0 and later versions.

[SebHeuze]

Yes you are right, my mistake was because the project that had the issue forced the log4j version so I had the compatibility issue with 2.3.9