FIX UserWarning when using ASGI mode

[ZinkLu]

get rid of warning when using ASGI server to run sanic.

INFO:     Started server process [69183]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
./sanic/base.py:35: UserWarning: Setting variables on Sanic instances is deprecated and will be removed in version 21.9. You should change your Sanic instance to use instance.ctx._asgi_app instead.
  warn(
...

[codecov]

Codecov Report

Merging #2091 (1c0da57) into main (8f06d03) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #2091   +/-   ##
=========================================
  Coverage   92.133%   92.133%           
=========================================
  Files           38        38           
  Lines         3470      3470           
  Branches       572       572           
=========================================
  Hits          3197      3197           
  Misses         185       185           
  Partials        88        88           

Impacted Files	Coverage Δ
sanic/app.py	92.623% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8f06d03...1c0da57. Read the comment docs.