explicitly discourage use of `@ecs_compatibility` instance variable, since
it is not clear how Logstash Core will implement its ECS-Compatibility mode

Modifies API to make the ECS Schema major version the primary unit to target,
in order to provide a path to support the ECS Schema v2 that is targeted to
release along-side Elastic Stack 8.0

Plugin implementors are now supplied with `LegacyAdapter#ecs_compatibility`
that produces a `Symbol` representing the desired ECS-Compatibility mode
(either `:disabled` or a v-prefixed integer e.g., `:v1`).

Plugin configuration in a pipeline definition now can specify `disabled`
or a v-prefixed integer (e.g., `v1`).