Fix conflicting docs for /tekton/steps API Compatibility.

Previously the developer docs listed that /tekton/steps was in scope for
the API compatibility policy, where-as the [user
docs](https://github.com/tektoncd/pipeline/blob/aceb5880f009884be14f9a7409f428cc84157a75/docs/tasks.md#reserved-directories)
did not say it was in scope.

After discussion in
[PR #4352](#4352 (comment)),
we have come to agreement that this is **not** in scope.

While this change in itself is not a breaking change, this opens the
door for breaking changes (such as #4352). Our stance is that users
should use the [`$(steps.step-<step-name>.exitCode.path)`
syntax](https://github.com/tektoncd/pipeline/blob/aceb5880f009884be14f9a7409f428cc84157a75/docs/tasks.md#accessing-steps-exitcode-in-subsequent-steps) to
avoid breaking changes.

docs/developers/README.md

@@ -202,7 +202,6 @@ The following directories are covered by the
 relied on for stability:
 
 - `/tekton/results`
-- `/tekton/steps`
 
 All other files/directories are internal implementation details of Tekton -
 **users should not rely on specific paths or behaviors as it may change in the