Update POM parent and Maven + warn if older Maven is used to build a Maven plugin (as impacted by MNG-7055)

Update plugin build:

• update parent to 36
• update maven to 3.8.5 (reasoning below)
• up the plugin to Java8

This plugin along with m-install-p versions 3.x line (3.0.0-Mx) does not fully work (on maven-plugin packaging) with any so far released Maven 3.x line (see MNG-7055). The ONLY maven 3.x they will be usable with is upcoming 3.9.0, that has fix for MNG-7055 backported, but OTOH, it is already Java8, so no point in keeping the plugin on anything below.
In case an older Maven version is used for deploying a maven-plugin, a warning will be displayed:

[WARNING] 
[WARNING] You are about to deploy a maven-plugin using Maven 3.8.9.
[WARNING] This plugin should be used ONLY with Maven 3.9.0 and newer, as MNG-7055
[WARNING] is fixed in those versions of Maven only!
[WARNING]