Update Azure AD instructions in generic-oauth.md (#20091)

Update Azure AD instructions to follow heading changes in Azure Portal.

> In the legacy experience, an app had Keys page. In the new experience, it has been renamed to Certificates & secrets. In addition, Public keys are referred to as Certificates and Passwords are referred to as Client secrets.

Source: https://docs.microsoft.com/en-us/azure/active-directory/develop/app-registrations-training-guide#keyscertificates--secrets

I verified by logging in Azure AD and walking through the steps.

docs/sources/auth/generic-oauth.md

@@ -156,11 +156,11 @@ allowed_organizations =
 
 5.  Note down the "Application ID", this will be the OAuth client id.
 
-6.  Click "Settings", then click "Keys" and add a new entry under Passwords
-    - Key Description: Grafana OAuth
-    - Duration: Never Expires
+6.  Click "Certificates & secrets" and add a new entry under Client secrets
+    - Description: Grafana OAuth
+    - Expires: Never
 
-7.  Click Save then copy the key value, this will be the OAuth client secret.
+7.  Click Add then copy the key value, this will be the OAuth client secret.
 
 8.  Configure Grafana as follows: