Deprecate old analytics/ code

[widal001]

Summary

Remove old code and documents in the analytics/ codebase that has been replaced by our shift to Metabase.

This will require some investigation but the general things we can probably remove are:

• The metrics/ sub-package (previously used for calculating metrics before they were posted to Slack)
• CLI entrypoints and Makefile commands related to slack reporting
• Dependencies that were only used by those sections of the codebase
• Old documentation that references the slack-based reporting.

Acceptance criteria

• Sprint and delivery reports are no longer posted to slack
• The sections of the codebase that supported slack-based reporting have been removed
• Dependencies that were only used for slack-based reporting have been removed
• Documentation has been updated to reflect a Metabas-first approach to analytics

[widal001]

We may want to make #2130 a sub-issue