[CI] Improve BWC Tests and coverage

[dbwiddis]

Is your feature request related to a problem?

Our BWC tests are limited and essentially were the minimum required for the feature they were added with. We need to increase coverage and capability of the tests.

What solution would you like?

• Currently the old version is fixed to 2.12 (for the single feature we test). We need to test a range of previous versions.
• Currently rolling upgrades are disabled on major version changes. We need to test "latest 2.x" vs. 3.x rolling upgrade.
• Currently tests run without security plugin. This makes it impossible to use that plugin API to register a User to accurately test BWC on indices related to the User field (I'm looking at you, Add user mapping to Workflow State index #705). Tests should run with and without security.

What alternatives have you considered?

Manually performing one-time BWC checks when introducing a new feature and hoping you covered all the situations (which you probably didn't.)

Do you have any additional context?

It may be useful to work on #555 at the same time as this issue.

See this commit for a failed attempt to fix some of the above issues existing tests.