Improve HTTP/2 ALPN negotiation error message in Kestrel.

[adityamandaleeka]

Add actionable guidance to EndPointHttp2NotNegotiated error message, mentioning ALPN negotiation and suggesting either fixing the client configuration or enabling HTTP/1.1 as a fallback.

Fix #44241

[Copilot]

Pull request overview

This PR enhances the error message for HTTP/2 ALPN negotiation failures in Kestrel to provide more actionable guidance to developers. The update addresses issue #44241 by helping users understand what went wrong and how to resolve the issue.

Key Changes

• Updated the EndPointHttp2NotNegotiated error message to explain the ALPN negotiation requirement and suggest two remediation options
• Minor whitespace cleanup in XML comments (trailing spaces removed)

[JamesNK]

Would it be useful to have an aka.ms link here that links to our docs with more information?

[adityamandaleeka]

I'm not sure we have an existing page that is well-suited for this error. If we make one later we can add a link here, but for now I don't think it's worth blocking this PR for. Only 1 of the other 200-ish error messages in this file has a link in it and there's not much signal to indicate that this specific one is extra special... this change is just making the error message a bit more helpful.