[Request]: Remove secret-token from Serverless docs

[colleenmcginnis]

Description

Remove mentions of secret-token from the Serverless docs.

Resources

https://github.com/elastic/apm-managed-service/issues/1131

Which documentation set does this change impact?

Serverless only

Feature differences

N/A

What release is this request related to?

N/A

Collaboration model

The documentation team

Point of contact.

Main contact: @simitt

Stakeholders: N/A