Adapt AsyncAPI version in Solace example

Solace Binding Protocol has been included with release 2.3.0 of AsyncAPI,
with:
- asyncapi/spec@88abbe0
- asyncapi/spec#666

Thus, specification example should indicate at least this version.

While here, let's favor last released version 2.4.0, as suggested by @fmvilas

---

Noticed by working on Solace Protocol support on https://bump.sh/asyncapi

solace/README.md

@@ -113,7 +113,7 @@ channels:
       eventType:
         schema:
           type: string
-asyncapi: 2.0.0
+asyncapi: 2.4.0
 info:
   title: HRApp
   version: 0.0.1
@@ -156,7 +156,7 @@ channels:
       eventType:
         schema:
           type: string
-asyncapi: 2.0.0
+asyncapi: 2.4.0
 info:
   title: HRApp
   version: 0.0.1