[Repo Assist] Change HTTP response default encoding from ISO-8859-1 to UTF-8

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Implements the fix requested in #1251 — changes the HTTP response default encoding from ISO-8859-1 to UTF-8 to match System.Net.Http.HttpClient behaviour.

This is a clean rebase of #1665 on the current main branch (post-8.1.0 release). Please close #1665.

Closes #1251

Root Cause

HttpEncodings.ResponseDefaultEncoding was set to ISO-8859-1, following RFC 2616 §3.7.1. However, modern HTTP servers, browsers, and System.Net.Http.HttpClient all default to UTF-8 in practice when no charset is specified in the Content-Type response header. This caused garbled characters when fetching UTF-8 content from servers that don't include an explicit charset.

@dsyme endorsed this fix: "Agreed we should do the same as HttpClient and do this."

Change

• HttpEncodings.ResponseDefaultEncoding changed from Encoding.GetEncoding("ISO-8859-1") to Encoding.UTF8
• Updated XML doc comments across the type providers (XmlProvider, JsonProvider, CsvProvider, HtmlProvider) to reflect the new default
• Updated the test HttpEncodings.ResponseDefaultEncoding returns ISO-8859-1 to reflect the new value
• Added ## 9.0.0 section in RELEASE_NOTES.md (this is a breaking change: existing code relying on ISO-8859-1 default may need to set responseEncodingOverride)

The existing responseEncodingOverride parameter remains available for callers who need to force a specific encoding, including ISO-8859-1.

Test Status

• ✅ Build: succeeded
• ✅ All tests passed (FSharp.Data.Core.Tests, FSharp.Data.Tests, FSharp.Data.DesignTime.Tests, FSharp.Data.Reference.Tests)
• ✅ Fantomas format check: passed
• 1 test skipped (network test blocked by sandbox firewall — pre-existing infrastructure issue)

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