docs(webapp): trim server-changes note to one behavior-level line

Author: nicktrn

.server-changes/plan-aware-compute-migration.md

@@ -3,8 +3,4 @@ area: webapp
 type: feature
 ---
 
-Gradually route a configurable percentage of free (then paid) organizations onto
-the compute backing at trigger time, with a per-org exclusion and an admin kill
-switch. Controlled by the `computeMigrationEnabled`, `computeMigrationFreePercentage`,
-and `computeMigrationPaidPercentage` feature flags and the `COMPUTE_BACKING_MAP`
-env var.
+Gradually roll out a new run execution backend to a configurable percentage of organizations.