Release Metafacture core 5.6.0

[dr0i]

Follows up #476.

Next release following metafacture-core-5.5.0. Current HEAD at 1860026.

• update dependencies if these are promising to not break much (Update dependencies #489).
• test release candidate (see Update dependencies #489).
• release on maven central
• release on github
• update flux-commands
• write blog post, see PR
• toot
• write to email list

[dr0i]

All channels are up to date. Closing.