Add warn for selecting limit reduction with groups (not implemented)

[NathanDissoubray]

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?

What kind of change does this PR introduce?

Disable group selection for limit reduction, introduced by powsybl/powsybl-core#3802

What is the current behavior?

There is no implementation to support this in OLF.

What is the new behavior (if this is a feature change)?

The implementation is to disable it for now. It is supposed to be developped for the release in June. This is a temporary warn message for users for the march release.

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)

Other information:

[NathanDissoubray]

Wrong base branch, and rebasing to sensi_operator_strategy_creates conflict from main, I'll just close this, create a new branch and cherry pick the commit.