Diviner: Configuring Outbound Mail: Remove incorrect Gmail references

Summary:
Configuring SMTP for Gmail as described will trigger
"535-5.7.8 Username and Password not accepted. Learn more at 535 5.7.8 https://support.google.com/mail/?p=BadCredentials"
GMail only supports OAuth and 2FA/App Passwords these days; the "less secure apps" option (which would be needed for the described SMTP setup) has been removed.

Test Plan:
Follow the docs; set up `cluster.mailers` config setting in Phorge; run `./bin/mail send-test --to admin --subject subject < ~/foo` and fail.
Optionally, watch https://www.youtube.com/watch?v=Y_u5KIeXiVI for your personal corporate excitement.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25911

src/docs/user/configuration/configuring_outbound_email.diviner

···
 | SendGrid | Medium | Cheap | Yes | Email | |
 | Twilio | Easy | Cheap | No | SMS | Recommended |
 | Amazon SNS | Easy | Cheap | No | SMS | Recommended |
-| External SMTP | Medium | Varies | No | Email | Gmail, etc. |
+| External SMTP | Medium | Varies | No | Email | |
 | Local SMTP | Hard | Free | No | Email | sendmail, postfix, etc |
 | Custom | Hard | Free | No | All | Write a custom mailer. |
 | Drop in a Hole | Easy | Free | No | All | Drops mail in a deep, dark hole. |
···
 | Inbound | Requires Configuration
 |---------|
 
-You can use this adapter to send mail via an external SMTP server, like Gmail.
+You can use this adapter to send mail via an external SMTP server.
 
 To use this mailer, set `type` to `smtp`, then configure these `options`:
 
···
   - `port`: Optional int. The port to connect to on your SMTP server.
   - `user`: Optional string. Username used for authentication.
   - `password`: Optional string. Password for authentication.
-  - `protocol`: Optional string. Set to `tls` or `ssl` if necessary. Use
-    `ssl` for Gmail.
+  - `protocol`: Optional string. Set to `tls` or `ssl` if necessary.
   - `message-id`: Optional bool. Set to `false` if Phorge will not be
     able to select a custom "Message-ID" header when sending mail via this
     mailer. See "Message-ID Headers" below.