Define a default image update policy and cadence for GitLab SaaS Runners on Linux

Overview

The scope of this issue is to define a policy and process to ensure a regular review and update of the default container image used on the GitLab SaaS Runners on Linux.

Goal(s)

1. This change and the planned improvements to the default container image maintenance process for GitLab SaaS Runners on Linux.
2. ..

Items to include in the policy and process

1. Image review cadence timing.
2. Notification process to notify customers of a new image.