…elastic#212817)

**Epic:** elastic#174168
**Partially addresses:**
elastic#202078,
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:

- Creates a new test plan for prebuilt rule upgrade notifications on the
Rule Management, Rule Details, and Rule Editing pages. The filename is
`prebuilt_rule_upgrade_notifications.md`.
- Extracts the existing scenarios for upgrade notifications on the Rule
Management page from `prebuilt_rule_upgrade_without_preview.md` to
`prebuilt_rule_upgrade_notifications.md`. Also, updates them according
to the most recent UI behavior.
- Adds new scenarios for upgrade notifications on the Rule Details page
to `prebuilt_rule_upgrade_notifications.md`.
- Adds new scenarios for upgrade notifications on the Rule Editing page
to `prebuilt_rule_upgrade_notifications.md`.

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

(cherry picked from commit da0480b)