[CI] Improve BWC Tests and coverage

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, #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.

Start with the existing BWC tests and review the failed attempt in commit d071e3162e181703d50db9c5742610a11c18fa0e, then inspect the related work in #555. Done means covering a range of previous versions, testing latest 2.x to 3.x rolling upgrades, and running coverage with and without the security plugin.