Deprecate EPP healthChecking CLI Flag

[danehans]

#966 introduced a CLI flag for Gateway<>EPP active health checking. The flag defaults to false for backward compatibility purposes. I don't see a need for this flag to exist in the long term and should therefore be deprecated within a reasonable timeframe to allow support by implementations.

[danehans]

cc: @ahg-g @kfswain for how we should handle deprecations.