Move from logback to log4j2

[adriangonz]

Fixes #1007.

Changelog

• Add explicitly the spring-boot-starter package to handle excludes more cleanly
• Remove org.springframework:spring-web, which is already being added by spring-boot-starter-web
• Exclude spring-boot-starter-logging and add dependency on spring-boot-starter-log4j2

Notes

The only reason to add spring-boot-starter explicitly is that currently all the spring-boot-starter-* excludes are handled on spring-boot-starter-test, because it's the first one that appears. Since this one is also including spring-boot-starter, it makes sense to add it explicitly and specify the excludes there.

This is also the recommended way to do when the project combines multiple spring-boot-starter-* projects (e.g. here).

[seldondev]

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[adriangonz]

/cc @axsaucedo @cliveseldon