Update README to reflect new version usage only

Removed instructions for old versions of sbt-openapi-generator.

Author: wing328

README.md

@@ -6,22 +6,10 @@ A Sbt plugin to support the OpenAPI generator project.
 
 Add to your `project/plugins.sbt`:
 
-For new versions (7.14.0 or later), please use
-
 ```sbt
 addSbtPlugin("org.openapitools" % "sbt-openapi-generator" % "7.19.0")
 ```
 
-Ref: https://central.sonatype.com/artifact/org.openapitools/sbt-openapi-generator_2.12_1.0
-
-For old versions (e.g. 7.13.0), please use
-
-```sbt
-addSbtPlugin("org.openapitools" % "sbt-openapi-generator" % "7.13.0")
-```
-
-Ref: https://central.sonatype.com/artifact/org.openapitools/sbt-openapi-generator
-
 # Configuration
 
 Configuration based on project module is recommended way to separate specifications by modules.