is_multiple shouldn't require &mut self

[klnusbaum]

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• I have searched the open and rejected issues

Clap Version

4.1.4

Describe your use case

ArgGroup::is_multiple requires a &mut self, yet the function does not itself do any mutations to the ArgGroup. The &mut is unnecessary. It also means that when you have an immutable reference to an ArgGroup (like the one you would get from calling groups() on a command), you can't check it the group allows multiple options.

Describe the solution you'd like

We should remove the &mut from the function signature.

Alternatives, if applicable

No response

Additional Context

No response