Allow fork in limited cases

[ahopkins]

Supporting both fork and spawn is a PITA. For reasons that drove us towards the big change in v22.9, spawn should always be the default. This PR will not drop back to fork unless someone expressly chooses it. For more information on why, see: https://community.sanicframework.org/t/sanic-app-name-xxx-already-in-use-while-running-in-docker

[ahopkins]

This pull request has been mentioned on Sanic Community Discussion. There might be relevant details there:

https://community.sanicframework.org/t/sanic-app-name-xxx-already-in-use-while-running-in-docker/1106/7

[codecov]

Codecov Report

Base: 88.614% // Head: 88.512% // Decreases project coverage by -0.102% ⚠️

Coverage data is based on head (2626be3) compared to base (db39e12).
Patch coverage: 73.333% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2624       +/-   ##
=============================================
- Coverage   88.614%   88.512%   -0.103%     
=============================================
  Files           81        81               
  Lines         6684      6694       +10     
  Branches      1142      1143        +1     
=============================================
+ Hits          5923      5925        +2     
- Misses         524       529        +5     
- Partials       237       240        +3     

Impacted Files	Coverage Δ
sanic/worker/manager.py	90.506% <20.000%> (-2.305%)	⬇️
sanic/application/ext.py	100.000% <100.000%> (ø)
sanic/mixins/startup.py	90.990% <100.000%> (+0.102%)	⬆️
sanic/server/websockets/impl.py	37.557% <0.000%> (-0.922%)	⬇️

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