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

follow-up bug of https://github.com/adobe/aio-lib-java/issues/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?

Start by reviewing issue 182 and the dependency declarations in the top-level and AEM sub-package parent POMs. Determine how SLF4J versions are currently inherited and whether the proposed split keeps non-AEM consumers on 2.x while AEM bundles remain compatible with 1.x. Done means the dependency behavior is verified for both contexts.

Written by the indexing model from the issue text.