Release Metafacture core 7.0.0

[dr0i]

The maven central release was done manually, see #695.

This release follows 6.2.0 #580.

Following maintaining guidelines.

- [ ] update dependencies if these are promising to not break much

• test release candidates (more or less done, based on building locally at tag rc1)
• release on maven central
• release on github
• update metafacture-playground
• update flux-commands
• update [metafacture-fix' functions](https://github.com/metafacture/metafacture-documentation/blob/master/docs/fix/Fix-functions.md
• write blog post
• toot
• write to email list
• announce at metadaten.community

[TobiasNx]

Release on Github is published.