disable additionalProperties at top level of aggregated chart

[fstaudt]

invalid keys at top-level of values.yaml is only identified.

Invalid keys at first level of values.yaml are not identified as validation failures by IntelliJ IDEA.
They are only identified as validation failures by Gradle task validateHelmValues (thanks to flag unevaluatedProperties disabled at top level).

Flag additionalProperties should be disabled at top level of aggregated chart to ensure that those invalid keys are also identified by IntelliJ IDEA.
This flag should only be removed when unevaluatedProperties is supported by IntelliJ IDEA (part of JSON schema specification 2019-09).