Spring codegen script needs to be updated to generate classes for all stable versions of a library

Currently we only support the latest stable version of the library and drop support for an older version when a new one is available.
Most recent example can be seen in https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/2183 where the languages v1 code was dropped in favor of languages v2. The client libraries, on the other hand, support v1 and v2 side-by-side and we should be emulating the same behavior (See https://github.com/googleapis/google-cloud-java/tree/main/java-language).

Note that if we support multiple versions of the client libraires, we will also a starter for each version that's supported.

cc/ @blakeli0 @meltsufin @diegomarquezp