json: adjust definitions alias

JSON Schema suggests[1] using "$defs" rather than "definitions" as
the location for reusable definitions in a schema.  While, this
isn't strictly required, there is some implication that using it
lends to better schema validation error reporting[2].

Adjust all the schema files to use "$defs" rather than "definitions".

[1]: https://json-schema.org/understanding-json-schema/structuring#defs
[2]: https://github.com/orgs/json-schema-org/discussions/151

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4b23890950bd2ebb7a7de99fd911fb3c9e0a4dbd
diff --git a/schemas/satellite_controller.json b/schemas/satellite_controller.json
index 3a6c4af..f5e4d6e 100644
--- a/schemas/satellite_controller.json
+++ b/schemas/satellite_controller.json
@@ -1,6 +1,6 @@
 {
     "$schema": "http://json-schema.org/draft-07/schema#",
-    "definitions": {
+    "$defs": {
         "SatelliteController": {
             "title": "satellite controller configuration",
             "description": "The configuration used to add remote BMCs to a system",