Document Uri length limits removal in .NET 10 Preview 7

[Copilot]

This PR documents the breaking change where Uri length limits have been removed in .NET 10 Preview 7. Previously, Uri constructors and TryCreate methods limited Uri strings to around 65k characters, but these restrictions have now been lifted.

The documentation includes:

• New breaking change file: /docs/core/compatibility/networking/10.0/uri-length-limits-removed.md with complete details about the change
• Updated index: Added entry to the networking section in /docs/core/compatibility/10.0.md

The change allows creation of Uri instances with much larger paths, queries, and fragments, which enables scenarios like:

• Large data: URIs containing Base64-encoded binary data
• Extended query strings for inter-system communication
• Other use cases where Uri serves as an exchange type beyond HTTP requests

For users relying on Uri length restrictions for input validation, the documentation recommends implementing custom length checks before constructing Uri instances.

Fixes #47602.

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