Remove default project_id from p_ci_finished_build_ch_sync_event

What does this MR do and why?

This MR removes the project_id column default from the p_ci_finished_build_ch_sync_event table (as per the process outlined in the docs), and adds the sharding key to the docs YAML file. We do not need to care about columns_changing_default, since we never try to write the default -1 value to project_id.

• EE: true
• Changelog: changed

Part of #459997 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

n/a

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

n/a