[Security Solution] Test Plan for Rule Updates workflow (elastic#203331)

Partially resolves: elastic#202078

## Summary

This PR refactors the test plans involving the Installation and Upgrade
Workflows:

- breaks down the gigantic `installation_and_upgrade.md` file into three
files:
- `installation.md` covering exclusively prebuilt rule install workflows
    - `upgrade.md` covering exclusively prebuilt rule upgrade workflows 
- `package_installation_and_upgrade.md` which covers scenarios for the
installation and upgrades of the Prebuilt Rules package itself.

The *Upgrade of Prebuilt Rules* (`upgrade.md`) was modified as follows:

- expanded to cover scenarios detailed in the linked ticket (`Upgrading
prebuilt rules one-by-one without preview` and `Upgrading prebuilt rules
in bulk` in the Rule Updates table, including cases with and without
conflicts, as well as a mixture of both)
- internally refactored so that sections that will become legacy with
the release of Milestone 3 were pushed to the end and marked as pending
deletion once the feature is live.
- from point above, the test plan now is in a better position to add the
scenarios for the Three-Way-Diff upgrade workflow. cc @maximpn
@nikitaindik


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

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Co-authored-by: Maxim Palenov <maxim.palenov@elastic.co>