Remove opinions from Rfc2898DeriveBytes docs

[bartonjs]

• Remove the "recommended minimum" from the API documentation. The correct answer is too complicated for API docs.
• In the sample that references the "default" value (which is from an Obsolete constructor) call it instead the "legacy default".

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones

[Copilot]

Pull request overview

This PR updates the API reference documentation for System.Security.Cryptography.Rfc2898DeriveBytes to remove prescriptive guidance about iteration-count minimums and to clarify that the sample’s referenced constructor defaults are legacy behavior (from an obsolete constructor).

Changes:

• Removed “minimum recommended number of iterations is 1000” statements from multiple Remarks sections in Rfc2898DeriveBytes.xml.
• Removed the iteration-count recommendation from the IterationCount property’s Remarks so the section is no longer opinionated.
• Updated the C# snippet comment to describe the “legacy default” iteration count rather than calling it the “default”.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
xml/System.Security.Cryptography/Rfc2898DeriveBytes.xml	Removes iteration-count recommendation language from constructor and property documentation.
snippets/csharp/System.Security.Cryptography/Rfc2898DeriveBytes/Overview/rfc28981.cs	Updates sample commentary to refer to the obsolete-constructor behavior as a “legacy default”.