Merge maint: retire cost/congestion chaos scenarios

Bring up the retirement of the six cost-based parent-selection chaos
scenarios (cost-reeval, cost-avoidance, cost-stability, depth-vs-cost,
mixed-technology, bottleneck-parent), which tested a decision the Docker
harness could not exercise reliably.

master already carries the equivalent sans-IO coverage in
src/proto/stp/tests (effective-depth cost selection, hysteresis, cost
degradation) and its transport-drop tests, so this keeps master's src
unchanged and takes only the scenario removals and the CI-list, README and
scenario-comment updates from maint.

.github/workflows/ci.yml

@@ -454,24 +454,6 @@ jobs:
           - suite: tcp-mesh
             type: chaos
             scenario: tcp-mesh
-          - suite: bottleneck-parent
-            type: chaos
-            scenario: bottleneck-parent
-          - suite: cost-avoidance
-            type: chaos
-            scenario: cost-avoidance
-          - suite: cost-reeval
-            type: chaos
-            scenario: cost-reeval
-          - suite: cost-stability
-            type: chaos
-            scenario: cost-stability
-          - suite: depth-vs-cost
-            type: chaos
-            scenario: depth-vs-cost
-          - suite: mixed-technology
-            type: chaos
-            scenario: mixed-technology
           - suite: congestion-stress
             type: chaos
             scenario: congestion-stress