oauth2 processor: make client-authentication-method compatible with Boot 2.5

[Kehrlann]

When consuming a Boot 3 compatible binding, we get an OAuth2 client-authentication-method: client_secret_basic or client_secret_post. But that value is not recognized by Spring Security < 5.5 (Boot < 2.5), causing very hard to debug issues at runtime (calls to the token endpoint fail because the authentication method is unknown).

This PR transforms Boot 3-compatible client-authentication-method into Boot 2.x-compatible ones, which work with all minors.