Make Granian the default backend and Uvicorn/Gunicorn explicit opt-in

[FarhanAliRaza]

Context

Reflex was intended to default to Granian in 0.9, but the current automatic selection still chooses the Uvicorn/Gunicorn backend when both packages are importable and REFLEX_USE_GRANIAN is unset. This is especially visible in upgraded environments that retain the old backend dependencies.

PR #6758 only updates the resulting warning. The underlying selection behavior should be fixed instead.

Proposed behavior

1. Default REFLEX_USE_GRANIAN to True.
2. Use Granian when REFLEX_USE_GRANIAN is unset or true, regardless of whether Uvicorn/Gunicorn happen to be installed.
3. Use the Uvicorn/Gunicorn backend only when REFLEX_USE_GRANIAN=0 and its required packages are installed.
4. Remove the Uvicorn fallback warning because that backend becomes fully opt-in.

Expected selection matrix

REFLEX_USE_GRANIAN	Result
Unset	Granian
1 / true	Granian
0 / false, Uvicorn/Gunicorn available	Uvicorn/Gunicorn
0 / false, required packages missing	Define and test an actionable failure or fallback behavior

Acceptance criteria

• Update backend-selection logic and the environment-variable default/documentation.
• Remove the obsolete Uvicorn fallback warning.
• Add unit coverage for every row in the selection matrix.
• Update user-facing documentation and add the appropriate news fragment.
• Confirm whether this ships in the next 0.9.x release or requires 0.10.0 before implementation.

Supersedes #6758.

[linear-code]

ENG-10677