Generate server stubs for PHP-SLIM not working for 3.x

Description

I tried to generate my server stubs with the PHP SLIM framework code and I get the following:

$ java -jar swagger-codegen-cli.jar generate -i https://petstore.swagger.io/v2/swagger.json -l slim -o /c/Projects/test
Exception in thread "main" java.lang.RuntimeException: Can't load config class with name slim Available: dart
aspnetcore
csharp
csharp-dotnet2
go
go-server
dynamic-html
html
html2
java
jaxrs-cxf-client
jaxrs-cxf
inflector
jaxrs-cxf-cdi
jaxrs-spec
jaxrs-jersey
jaxrs-di
jaxrs-resteasy-eap
jaxrs-resteasy
micronaut
spring
nodejs-server
openapi
openapi-yaml
kotlin-client
kotlin-server
php
python
python-flask
r
ruby
scala
scala-akka-http-server
swift3
swift4
swift5
typescript-angular
typescript-axios
typescript-fetch
javascript

        at io.swagger.codegen.v3.CodegenConfigLoader.forName(CodegenConfigLoader.java:31)
        at io.swagger.codegen.v3.cli.SwaggerCodegen.main(SwaggerCodegen.java:96)
Caused by: java.lang.ClassNotFoundException: slim
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
        at java.base/java.lang.Class.forName0(Native Method)
        at java.base/java.lang.Class.forName(Class.java:375)
        at io.swagger.codegen.v3.CodegenConfigLoader.forName(CodegenConfigLoader.java:29)
        ... 1 more

As you can see, there is no slim option in the list. The documentation says it should work.
If I run the same command against my 2x jar the command succeeds.
The problem is that 2x does not support Open API 3.0.3 and I need to support 3.0.3.

Swagger-codegen version

3.0.34

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Start at CodegenConfigLoader.java and SwaggerCodegen.main, which appear in the failure trace, and compare the documented PHP-SLIM generator with the available generator list. Reproduce the 3.0.34 command against the Petstore OpenAPI document; done means the slim option is recognized and server stubs generate successfully for OpenAPI 3.0.3.