…astic#213434)

**Epic:** elastic#174168
**Partially addresses:**
elastic#202079,
elastic#210358

## Summary

We started to rework and introduce functional changes to our existing
test plans for prebuilt rule customization, upgrade, and export/import
workflows.

Specifically, this PR:

- Restructures the test plan, introduces a more fine-grained list of
sections.
- Rewrites almost all the existing scenarios. In most cases it boils
down to splitting a scenario into 2+ more specific scenarios, where each
describes _exactly_ what happens in the GIVEN and THEN sections. This is
very important, as it:
- makes these scenarios ready to be implemented right away: 1 scenario =
1 test to write
  - helps with ensuring that we covered edge cases
- Adds new scenarios for handling missing base versions according to
[elastic#210358](elastic#210358).
- Adds a placeholder section for licensing scenarios according to
[elastic#11502](elastic/security-team#11502).
- Removes scenarios for the `overwrite` request parameter - this is
common importing logic which is not related to prebuilt rules.
- Addresses my own comments from [this
review](elastic#206893 (review)).

The new test plan should be in line with the changes discussed in
elastic#210358.

(cherry picked from commit a9106c6)