[Many projects] Upgrade to Experience Manager 15

Upgrade Forge projects at https://github.com/bloomreach-forge to Experience Manager 15.0.0 / 15.1 /15.2.x / 15.3.x

Documentation is available at 
https://xmdocumentation.bloomreach.com/library/upgrade-14-to-15/upgrade-to-brxm-15.html

Most notable changes:

• Java 11, in subpage, also CMS-12371 
• Wicket 9, in subpage, also https://jira.bloomreach.com/browse/PRODDOCS-446

In code base:

• First check outstanding MRs, dependabot does a lot of security version bumps automatically. If needed, merge, possibly do a release (see how to release section).
• Make a 'support/CURRENTMAJOR.x' maintenance branch out of current 'develop' then do the upgrade on 'develop' or on separate upgrade branch, based on 'develop'
• In pom.xml, bump versions not only of Bloomreach XM but check latest for every dependency.
• In pom.xml, check if Maven plugin versions can be inherited of bumped.
• In pom.xml, update Maven URLs from onehippo.com to bloomreach.com, in <repositories> and <distributionManagement>. See ENT-5911.
• If present, remove file .travis.yml since the building on travis-ci.org is ceased. Also the possible Build Status reference in the README
• If present, upgrade (or create new from archetype) demo project in the submodule
• Document at the project's doc pages, use latest Forge Maven Skin 3.2.1 if you use it.
• Release as (CURRENTMAJOR+1).0.0, see 'How to release with git flow' https://bloomreach-forge.github.io/using-git-flow.html
• Document at https://bloomreach-forge.github.io/project-list.html 

Projects that are known to be used and have precedence: 

• JCR POJO Binding 
• Content Ex/Im
• IP Filter plugin
• Breadcrumb
• Embargo
• Localization Shortcut
• External Document Picker
• HST Spring Security Support
• OAI-PMH Provider
• Document Translation Picker
• Document Commenting
• brut
• Properties Plugin
• Reset Password (FORGE-429)
• Page Management Support (FORGE-430)
• Feeds
• Apache Camel Hippo Events Support (FORGE-439)
• Folder Context Menus (FORGE-437)
• Hippo Utilities (FORGE-436)
• IFrame Perspective (FORGE-433)
• Review Workflow (FORGE-438)
• Templating Support
• Essentializer (FORGE-435)