refactor: change oidc callback URL

BREAKING CHANGE: This patch changes the OpenID Connect and OAuth2 ("Sign in with Google, Facebook, ...") Callback URL from `http(s)://<kratos-public>/self-service/browser/flows/strategies/oidc/<provider>` to `http(s)://<kratos-public>/self-service/methods/oidc/<provider>`. To apply this patch, you need to update these URLs at the OAuth2 Client configuration pages of the individual OpenID Conenct providers (e.g. GitHub, Google).