doc: Update user facing docs to describe what has been migrated as a partof new agent authorization strategy

MR:

• Update workspace docs to describe migration add... (!160348 - closed)
• Update workspace release post to describe migra... (gitlab-com/www-gitlab-com!135519 - merged)

Description

Update the user-facing documentation to highlight how the migration was carried out as a part of the general rollout of the new authorisation strategy.

Acceptance Criteria

• Update the user facing docs with migration details

Technical Requirements

NA

Design Requirements

NA

Impact Assessment

NA

User Story

NA