Switch from `consecutive errors` to `error rate` traffic control strategy

[bolshakov]

Error rate is a more universal approach that enables better circuit breaker decisions across varying traffic patterns. Proposed new default configuration:

• window_size: 60 (seconds)
• threshold: 0.5 (50% error rate)

Before introducing this change, we'll have a deprecation period in the v5.x series. To avoid annoying deprecation warnings, users will be able to opt in to v6 defaults while still using v5:

Stoplight.configure(defaults_version: :v6) do 
  ...
end

Acceptance Criteria

• A warning should be issued if users rely on the default settings (consecutive errors strategy)

# Assumes consecutive errors strategy, therefore produces a warning
Stoplight('legacy-service', threshold: 5)

# Assumes consecutive errors strategy, therefore produces a warning
Stoplight('new-service', window_size: 60)

• When a user uses custom window_size/threshold settings, the warning should not be issued
• Users should be able to opt in to use v6 defaults early and not see the warning.

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We'd love your feedback on this decision!

• Does the error rate approach work for your use cases?
• Are the proposed defaults (window_size: 60, threshold: 0.5) reasonable for your applications?
• Do you have concerns about the migration path or the opt-in mechanism?

Please share your thoughts and any edge cases we should consider.

[bolshakov]

@Lokideos thoughts?