refactor: use `UeberauthOidcc` as a library

[paulswartz]

@yordis not :100: sure this is what you meant in https://github.com/erlef/oidcc/discussions/263, but I gave it a shot anyways.

Other than requiring users to update to Erlang 26 and possibly changing some error messages, this should be backwards compatible with existing Ueberauth.Strategy.Google configurations.

Benefits:

• native support for security features such as PKCE and nonces
• less OIDC / OAuth2 specific code to test/maintain

Cons:

• bumps the minimum Elixir/Erlang versions