fix(http11): disable HTTP/2 fallback for -pr http11 (Fixes #2240)

[wh0amibjm]

Fixes #2240

Summary:

• When Protocol is forced to HTTP/1.1 ("-pr http11"), set retryablehttp-go option DisableHTTPFallback to prevent switching to the internal HTTP/2 fallback client on malformed HTTP/2 responses.

Note:

• This PR depends on projectdiscovery/retryablehttp-go PR httpx gets stuck - No output #527 (adds Options.DisableHTTPFallback).
• Until that PR is merged and a module version is available, CI may fail due to the missing field.

Tests (local, with temporary module replace):

• go test ./common/httpx -count=1

/claim #2240

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