Add support for NiFi 1.15

[fhennig]

Description

• Support for 1.15.0 was added and support for 1.13.2 removed.
• The now required property nifi.sensitve.props.key can now be set via a secret.
• The property is mounted as an enviroment variable into the container, it is added to the `nifi.properties by a script. For this to work, the docker image had to be modified, please se related pull request in that repository: Added update_config script to nifi (For NiFi 1.15) docker-images#27
• A few new config settings were added and old ones removed.

Review Checklist

• Code contains useful comments
• (Integration-)Test cases added (or not applicable)
• Documentation added (or not applicable)
• Changelog updated (or not applicable)
• Cargo.toml only contains references to git tags (not specific commits or branches)

[maltesander]

Rustfmt is complaining. Otherwise i would say we remove 1.13.2 completly?
Edit: Didnt see its still draft:)

[maltesander]

Just minor things.

[maltesander]

Changelog link missing

[maltesander]

LGTM.