fix: force HTTP1 for upstream connections for WS and WSS backends

[zhaohuabing]

This PR forces forces HTTP/1.1 upstream connections instead of negotiating HTTP/2 for ws and wss Backend appProtocols. This change avoids compatibility issues with WebSocket backends that do not support RFC 8441 extended CONNECT.

Fixes: #8693

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	22bf777
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/69d7025b8d5afd000894c3af

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.35%. Comparing base (082b038) to head (22bf777).

Files with missing lines	Patch %	Lines
internal/ir/xds.go	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8699      +/-   ##
==========================================
- Coverage   74.36%   74.35%   -0.01%     
==========================================
  Files         244      244              
  Lines       38768    38802      +34     
==========================================
+ Hits        28828    28853      +25     
- Misses       7947     7952       +5     
- Partials     1993     1997       +4     

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