Update Complex.ToString to reflect formatting in .NET 8+

[jeffhandley]

Per dotnet/docs#41835, the format from Complex.ToString changed in .NET 8 from (a, b) to <a; b>.

Summary

Fixes dotnet/runtime#93454

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Tagging subscribers to this area: @dotnet/area-system-numerics

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Learn Build status updates of commit d4ca96f:

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File	Status	Preview URL	Details
xml/System.Numerics/Complex.xml	✅Succeeded	View

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[BillWagner]

This LGTM. Let's

[gewarren]

@jeffhandley It seems like we should keep the old and new syntax and annotate them with the versions they're applicable to.

[jeffhandley]

OK, I can send a follow up PR, @gewarren. Would the following wording be good? Please feel free to offer refinement to it.

Suggested change

	The default string representation of a complex number displays the number using its Cartesian coordinates in the form `<`*a*`;` *b*`>`, where *a* is the real part of the complex number, and *b* is its imaginary part. Both *a* and *b* are formatted using the general format specifier ("G") and the conventions of the current system culture.
	The default string representation of a complex number displays the number using its Cartesian coordinates in the form `<`*a*`;` *b*`>`, where *a* is the real part of the complex number, and *b* is its imaginary part. Both *a* and *b* are formatted using the general format specifier ("G") and the conventions of the current system culture. In .NET 7 and earlier, the form `(`*a*`, `*b*`)` was used.

[gewarren]

Suggested change

	The default string representation of a complex number displays the number using its Cartesian coordinates in the form `<`*a*`;` *b*`>`, where *a* is the real part of the complex number, and *b* is its imaginary part. Both *a* and *b* are formatted using the general format specifier ("G") and the conventions of the current system culture.
	The default string representation of a complex number displays the number using its Cartesian coordinates in the form `<`*a*`;` *b*`>` (or `(`*a*`, `*b*`)` in .NET 7 and earlier versions), where *a* is the real part of the complex number, and *b* is its imaginary part. Both *a* and *b* are formatted using the general format specifier ("G") and the conventions of the current system culture.

@jeffhandley This would be my recommended wording.