Prepare release v0.3.0

[olperr1]

Please check if the PR fulfills these requirements

• [X] 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)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Release

What is the current behavior?

powsybl-optimizer is in version 0.3.0-SNAPSHOT.

What is the new behavior (if this is a feature change)? powsybl-optimizer is in version 0.4.0-SNAPSHOT.

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

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

DO NOT squash the commit!

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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