oAuth 2 clients may break with SpringBoot 3

[gm2552]

Depending on the value set by an oAuth2 ProvisionedService for the ClientAuthenticationMethod field, SpringBoot 3 apps may break when acting as an oAuth2 client. The value of this field gets mapped verbatim to the Spring spring.security.oauth2.client.registration.[registrationId].client-authentication-method property via the spring-cloud-bindings library when using service binding.

Internally, this value gets converted to a ClientAuthenticationMethod instance. This works fine for SpringBoot 2.7.x and below as the class maps the following Strings to defaulted instances (Note: the values allowed in the ClientRegistration resource are deprecated):

	@Deprecated
	public static final ClientAuthenticationMethod BASIC = new ClientAuthenticationMethod("basic");
	public static final ClientAuthenticationMethod CLIENT_SECRET_BASIC = new ClientAuthenticationMethod(
			"client_secret_basic");
	@Deprecated
	public static final ClientAuthenticationMethod POST = new ClientAuthenticationMethod("post");
	public static final ClientAuthenticationMethod CLIENT_SECRET_POST = new ClientAuthenticationMethod(
			"client_secret_post");
	public static final ClientAuthenticationMethod CLIENT_SECRET_JWT = new ClientAuthenticationMethod(
			"client_secret_jwt");
	public static final ClientAuthenticationMethod PRIVATE_KEY_JWT = new ClientAuthenticationMethod("private_key_jwt");
	public static final ClientAuthenticationMethod NONE = new ClientAuthenticationMethod("none");
```	
In boot 3, the deprecated fields are no longer available meaning the clientAuthenticationMethod does not map to valid instance if the deprecated values are used for `ClientAuthenticationMethod` field of the  `ProvisionedService`.  This is causing the oAuth2 process to fail when the AuthServer is redirected back to the application's callback URL.

Simply asking a `ProvisionedService` to update the value of this field may break other apps that are depending on the same `ProvisionedService` instance.

[Kehrlann]

Related: #89