Update correctly the generator targetV when its remote regulating Terminal is removed

[alicecaron]

Describe the current behavior

When a generator remote regulating terminal is removed, the generator switches to local voltage regulation but the targetV is not updated consequently.

Describe the expected behavior

When a generator remote regulating terminal is removed, the generator switches to local voltage regulation and the targetV is updated consequently.

• if the bus is not already regulated then the generator targetV is updated with the nominal voltage in per unit
• else the targetV is set with the existing targetV of other regulating generators on this bus (for consistency).

A ReportNode should be created in each case so that the user may be notified of the change.

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

https://github.com/powsybl/powsybl-core/blob/main/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/RegulatingPoint.java#L142