VoltagePerReactivePowerControl extension removal

[olperr1]

⚠️ Draft status: This branch should be rebased on the msa/3618-refactor-voltage-regulation branch once #3777 is merged.
Only the last commit is not part of #3777.
Diff can be seen via this link.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• A PR or issue has been opened in all impacted repositories (if any)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Part of the voltage regulation refactoring

What is the current behavior?

StaticVarCompensator's slope attribute is stored in a dedicated extension VoltagePerReactivePowerControl.

What is the new behavior (if this is a feature change)?
With the voltage regulation refactoring, the slope is stored in the VoltageRegulation object. The VoltagePerReactivePowerControl extension is not needed anymore. This PR remove this extension.
Import/export backward-compatibility is supported:

• Older IIDM files containing the extension can still be read;
• It is possible to export the network in previous IIDM versions. In this case, the extension is written in the network file.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

TODO

Other information:

[sonarqubecloud]

Quality Gate failed

Failed conditions
3.1% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud