fix(providers): use client_secret_post token auth for LinkedIn (#5236)

* Send client_id and client_secret to linkedin

Linkedin now requires client_id and client_secret to be
sent in the oauth callback. Fixes #5220

* Update linkedin.ts

Co-authored-by: Thang Vu <thvu@hey.com>
Co-authored-by: Balázs Orbán <info@balazsorban.com>

packages/next-auth/src/providers/linkedin.ts

@@ -31,6 +31,9 @@ export default function LinkedIn<P extends LinkedInProfile>(
       params: { scope: "r_liteprofile r_emailaddress" },
     },
     token: "https://www.linkedin.com/oauth/v2/accessToken",
+    client: {
+      token_endpoint_auth_method: "client_secret_post",
+    },
     userinfo: {
       url: "https://api.linkedin.com/v2/me",
       params: {