Ensure the Backend Engineer Job Family performance indicators are up to date

Problem to solve

The Job Family performance indicators are part of the talent assesment program (https://about.gitlab.com/handbook/people-group/talent-assessment/#measuring-performance). As such, we should set clear expectations about what we're measuring performance against for a given role.

We currently have duplication and outdated content in the handbook - at least for the backend engineer job family - making it difficult or ambiguous to determine which performance indicators correspond to a Job Family.

Proposal

1. Ensure team members have a SSOT to look for their job family performance indicators (avoid duplication in the handbook).
2. Update Job Family performance indicators to match the new/updated indicators to use.
3. Create/improve Performance Indicator update process to ensure the job family stay in sync with them.