Kotlin nullability for Spring Boot 4 compatibility

For Kotlin WebClient and RestClient generated clients.

The current implementation causes a generated client like
```kt
open class ApiClient(protected val client: RestClient) {

    protected inline fun <reified I : Any, reified T: Any?> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
        return prepare(defaults(requestConfig))
            .retrieve()
            .toEntity(object : ParameterizedTypeReference<T>() {})

    }

    // ...
}
```
while that gives errors:

Author: hiddewie

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache

@@ -10,7 +10,7 @@ import org.springframework.util.LinkedMultiValueMap
 
 {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open class ApiClient(protected val client: RestClient) {
 
-    protected inline fun <reified I : Any, reified T: Any?> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
+    protected inline fun <reified I : Any, reified T: Any> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
         return prepare(defaults(requestConfig))
             .retrieve()
             .toEntity(object : ParameterizedTypeReference<T>() {})
@@ -32,7 +32,7 @@ import org.springframework.util.LinkedMultiValueMap
             }
         }
 
-    private fun <I> RestClient.method(requestConfig: RequestConfig<I>)=
+    private fun <I> RestClient.method(requestConfig: RequestConfig<I>) =
         method(HttpMethod.valueOf(requestConfig.method.name))
 
     private fun <I> RestClient.RequestBodyUriSpec.uri(requestConfig: RequestConfig<I>) =

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-webclient/infrastructure/ApiClient.kt.mustache

@@ -12,7 +12,7 @@ import reactor.core.publisher.Mono
 
 {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open class ApiClient(protected val client: WebClient) {
 
-    protected inline fun <reified I : Any, reified T: Any?> request(requestConfig: RequestConfig<I>): Mono<ResponseEntity<T>> {
+    protected inline fun <reified I : Any, reified T: Any> request(requestConfig: RequestConfig<I>): Mono<ResponseEntity<T>> {
         return prepare(defaults(requestConfig))
             .retrieve()
             .toEntity(object : ParameterizedTypeReference<T>() {})
@@ -34,7 +34,7 @@ import reactor.core.publisher.Mono
             }
         }
 
-    private fun <I> WebClient.method(requestConfig: RequestConfig<I>)=
+    private fun <I> WebClient.method(requestConfig: RequestConfig<I>) =
         method(HttpMethod.valueOf(requestConfig.method.name))
 
     private fun <I> WebClient.RequestBodyUriSpec.uri(requestConfig: RequestConfig<I>) =