OpenIDConnectAuth.discover: Only reset supported grant types when the server sends it

### Version

5.0.4

### Context

While using `OpenIDConnectAuth.discover`, I am setting `supportedGrantTypes` to be only `implicit`. In our specific scenario, we are using the OAuth2 handler only to validate JWT access tokens sent by clients, therefore we don't need any other grant type.

After discovery, the `OpenIDConnectAuth` resets `supportedGrantTypes` to `null`. When validating the configuration, `OAuth2Auth` falls back to the default, which is `auth_code, implicit`, leading to the following exception:

```
java.lang.IllegalStateException: Configuration missing. You need to specify [clientId]
at io.vertx.ext.auth.oauth2.OAuth2Options.validate(OAuth2Options.java:629) ~[vertx-auth-oauth2-5.0.4.jar:5.0.4]
at io.vertx.ext.auth.oauth2.impl.OAuth2AuthProviderImpl.(OAuth2AuthProviderImpl.java:70) ~[vertx-auth-oauth2-5.0.4.jar:5.0.4]
at io.vertx.ext.auth.oauth2.OAuth2Auth.create(OAuth2Auth.java:56) ~[vertx-auth-oauth2-5.0.4.jar:5.0.4]
at io.vertx.ext.auth.oauth2.providers.OpenIDConnectAuth.lambda$discover$1(OpenIDConnectAuth.java:149) ~[vertx-auth-oauth2-5.0.4.jar:5.0.4]
at io.vertx.core.impl.future.Composition.complete(Composition.java:40) ~[vertx-core-5.0.4.jar:5.0.4]
at io.vertx.core.impl.future.FutureBase.emitResult(FutureBase.java:68) ~[vertx-core-5.0.4.jar:5.0.4]
at io.vertx.core.impl.future.FutureImpl.completeInternal(FutureImpl.java:163) ~[vertx-core-5.0.4.jar:5.0.4]
at io.vertx.core.impl.future.FutureBase.emitResult(FutureBase.java:68) ~[vertx-core-5.0.4.jar:5.0.4]
```

although no clientId should be required, because it was never intended to be used.

A potential solution would be to reset the `supportedGrantTypes` only when the authorization server defines `grant_types_supported` the field in its response. Otherwise the configured grant types should be kept.

### Steps to reproduce

_No response_

### Do you have a reproducer?

_No response_