`core` : upgrade `slf4j` while still supporting aem customers

[francoisledroff]

follow-up bug of #182

Maybe leave the top-level slf4j dependency version to be 2.x,
but the AEM sub-package parent pom overrides it downgraded to 1.x?

This way third party, non-aem uses can keep up-to-date, but the AEM bundles still deploy correctly to that context. Thoughts?