Backport PR #12830 to 7.x: ecs-compatibility docs

[yaauie]

Backport subset of PR #12830 to 7.x branch. Original message:

Release notes

[rn:skip]

What does this PR do?

This change adds documentation about how to configure ECS Compatibility Modes at the plugin instance, pipeline, and global levels.

These docs are a 7.x-targeted backport of docs introduced in #12830 for 8.0, that have been re-worded from the perspective of Logstash 7.x to lead people toward the upgrade.

If a user wants to "lock in" pre-ECS behaviour in 7.x, they can do at different levels so by:

1. declaring ecs_compatibility => disabled on each individual plugin; OR
2. declaring pipeline.ecs_compatibility: disabled in each pipeline's definition (config/pipelines.yml or Central Management)
3. declaring pipeline.ecs_compatibility: disabled in config/logstash.yml to provide a global default for pipelines.

Why is it important/What is the impact to the user?

The transition to ECS being on-by-default with Logstash 8.0 is a potentially sharp edge, as the plugins that implement ECS Compatibility modes will operate differently than in their legacy versions when run in Logstash 8, unless specific effort is made to opt-out of the breaking change.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

[karenzone]

Nicely done! I left one comment in line. Otherwise, LGTM. 🎉