Release Metafacture core 6.1.0

[dr0i]

! Don't use this release - it has introduced bugs, see #527 (comment). !

Follows up #515.

• test release candidates
• release on maven central
• release on github
• release metafacture-fix 1.1.0
• release metafacture-playground 1.1.0
• update flux-commands
• copy metafacture-fix' functions-and-cookbook to metafacture-documentation/Fix-function-and-Cookbook.md
• write blog post, see PR
• toot
• write to email list
• announce at [metadaten.community](https://metadaten.community/t/neue-metafacture-releases-core-6-1-0-fix-1-1-0-playground-1-1-0/...

[dr0i]

Based on #541 is the sonatype SNAPSHOT release 6.1.0-rc1-SNAPSHOT and github packages 6.1.0-rc1-SNAPSHOT . See wiki/Maintainer-Guideline how to use it. Please test it and check mark:

• lobid-resources
• limetrans
• fix
• playground