Starter for Kotlinx Serialization Json is misnamed

[wilkinsona]

In #48076, the module was renamed from spring-boot-kotlin-serialization to spring-boot-kotlinx-serialization-json. The starter should have been renamed as well, becoming spring-boot-starter-kotlinx-serialization-json. We need to decide how and when we want to correct this:

1. With a breaking rename
   a. In 4.0.x
   b. In 4.1.x
2. Deprecating the old starter and introducing the new one
   a. In 4.0.x
   b. In 4.1.x

[philwebb]

We discussed this today and since we're do early in the 4.0.x releases, we're going to risk a quick rename with a special note in the release notes.