fix(exec): correct stale 0.8.0 reference in Uvicorn/Granian warning

[anxkhn]

## What / why

When the backend starts with Uvicorn (uvicorn and gunicorn installed,
`REFLEX_USE_GRANIAN` unset), Reflex warns:

> Using Uvicorn for backend as it is installed. This behavior will change in 0.8.0 to use Granian by default.

That message is stale in two ways:

- `0.8.0` shipped long ago (current release is 0.9.x), so it points users at a
  version that is already in the past.
- The switch never actually happened at 0.8.0. The very function that emits the
  warning, `should_use_granian()`, still returns `False` (Uvicorn) in this case,
  so the message contradicts the code that prints it.

The `0.8.0` string is a leftover from the Granian-as-default work that was moved
back to the sidelines and re-gated behind `REFLEX_USE_GRANIAN`; the target
version was never updated when the change was deferred.

## Change

Reword the single warning string to be version-agnostic and actionable, without
committing to a maintainer-owned target version:

> Using Uvicorn for backend as it is installed. Reflex will switch to Granian by default in a future release; set REFLEX_USE_GRANIAN=1 to opt in now.

`REFLEX_USE_GRANIAN=1` is a correct opt-in: `should_use_granian()` honors the env
var when it is set. No behavior change; only the string literal is different.

If you would rather name a concrete target version (e.g. the next major) instead
of "a future release", I am happy to adjust the wording.

## Tests

Added `test_warn_user_about_uvicorn_does_not_reference_past_version` in
`tests/units/utils/test_exec.py`. It unwraps the `@once` cache so the warning
fires, then asserts the message no longer contains `0.8.0` and still mentions
`Granian`. It fails against the old string and passes with the fix.

- `uv run pytest tests/units/utils/test_exec.py -q` -> 3 passed
- `uv run ruff check .` / `uv run ruff format .` -> clean
- `uv run pyright reflex tests` -> 0 errors

Added a `news/` bugfix fragment for the CHANGELOG check.

news fragment

news/6720.bugfix.md (placeholder PR number 6720). MUST be renamed to the real
PR number after the PR is opened, otherwise the CHANGELOG CI gate stays keyed to
the wrong number. Content:

Corrected the Uvicorn backend warning that claimed the switch to Granian as the default would happen in 0.8.0, a version that has already shipped; it now describes the change as coming in a future release and points to REFLEX_USE_GRANIAN=1 to opt in early.

[greptile-apps]

Greptile Summary

This PR updates the warning shown when Reflex selects Uvicorn.

• Removes the stale 0.8.0 reference.
• Documents REFLEX_USE_GRANIAN=1 as the current opt-in.
• Adds a focused warning-text test and a bugfix news fragment.

Confidence Score: 5/5

This looks safe to merge.

• The documented environment value parses as true and selects Granian.
• The test uses an API supported by the project’s Python versions and compatible with the @once wrapper.
• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
reflex/utils/exec.py	Replaces the obsolete version-specific Uvicorn warning with version-neutral Granian opt-in guidance.
tests/units/utils/test_exec.py	Adds a focused test that unwraps the cached warning function and checks the revised message.
news/6720.bugfix.md	Adds the configured bugfix fragment describing the warning correction.

_{Reviews (1): Last reviewed commit: "fix(exec): correct stale 0.8.0 reference..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

---

_{Comparing anxkhn:patch-18 (3edee85) with main (32cc257)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩