Fix incorrect UriBuilder.Scheme property documentation about valid schemes

[Copilot]

The UriBuilder.Scheme property documentation contained a misleading table that suggested only specific schemes (file, ftp, gopher, http, https, mailto, news) were valid. This was incorrect.

The actual implementation validates scheme names according to RFC 2396 via System.Uri.CheckSchemeName, which allows any valid URI scheme per the RFC specification, not just the predefined list shown in the documentation.

Before:
The remarks section showed a table listing specific "valid scheme names" that implied these were the only acceptable values.

After:
The remarks now accurately state: "The set accessor validates the scheme name for validity according to RFC 2396. The scheme name must begin with a letter and must contain only letters, digits, and the characters '.', '+', or '-'."

This change:

• Removes the misleading restriction to specific schemes
• Accurately describes the RFC 2396 validation that actually occurs
• Follows the same documentation pattern used elsewhere in the codebase (e.g., System.Uri)
• Allows developers to understand they can use custom schemes like x-custom-protocol or my-scheme as long as they comply with RFC 2396

Fixes #11626.

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[MihaZupan]

Thanks