fix: update Okta routes (#763)

the current routing for the Okta provider does not follow the standard
set by Okta, and as such doesn't allow for custom subdomains. this
update amends the routes to allow for customer subdomains, and also
aligns next-auth with Okta's documentation.

src/providers/okta.js

@@ -11,9 +11,9 @@ export default (options) => {
       client_secret: options.clientSecret
     },
     // These will be different depending on the Org.
-    accessTokenUrl: `https://${options.domain}/oauth2/v1/token`,
-    authorizationUrl: `https://${options.domain}/oauth2/v1/authorize/?response_type=code`,
-    profileUrl: `https://${options.domain}/oauth2/v1/userinfo/`,
+    accessTokenUrl: `https://${options.domain}/v1/token`,
+    authorizationUrl: `https://${options.domain}/v1/authorize/?response_type=code`,
+    profileUrl: `https://${options.domain}/v1/userinfo/`,
     profile: (profile) => {
       return { ...profile, id: profile.sub }
     },