[MNG-7532] Revert MNG-6931 deprecation since list shows no consensus on that

**[Romain Manni-Bucau](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=romain.manni-bucau)** opened **[MNG-7532](https://issues.apache.org/jira/browse/MNG-7532?redirect=false)** and commented

There are several threads on the dev list asking for the drop of slf4j and at least keeping a logging abstraction for not internal dev (= core can use slf4j but not mojo/extensions).

Work is being done to abstract plugin api so let's keep the deprecation/replacement of our Log API in this track and keep it the official way for now.

one of the multiple refs: https://www.mail-archive.com/dev@maven.apache.org/msg123452.html

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**Issue Links:**
- [MNG-6931](https://issues.apache.org/jira/browse/MNG-6931) Deprecate custom logging approach
(_**"fixes"**_)
- [MNG-7827](https://issues.apache.org/jira/browse/MNG-7827) Fix org.apache.maven.plugin.logging.Log deprecation message and AbstractMojo#getLog fallback
(_**"is duplicated by"**_)
- [MNG-7827](https://issues.apache.org/jira/browse/MNG-7827) Fix org.apache.maven.plugin.logging.Log deprecation message and AbstractMojo#getLog fallback

- [MNG-7839](https://issues.apache.org/jira/browse/MNG-7839) Plugin v4 API should ensure class realm are fully isolated from maven.core/extensions realms (classes+resources)

**Remote Links:**
- [GitHub Pull Request #793
](https://github.com/apache/maven/pull/793)