Support BWC using multiple older versions

[ohltyler]

Currently, the BWC scenarios in build.gradle are only supporting upgrades from the latest non-OpenSearch release (ODFE 1.13 / ES 7.10) to the current upcoming OpenSearch release (1.3). This should be changed to support all previous minor versions. For example, given the current developing version is 1.3, we should support:

7.10 -> 1.3
1.0 -> 1.3
1.1 -> 1.3
1.2 -> 1.3

At the time of this writing I don't believe any other plugin has implemented a clean way to do this yet. More details on the discussion of this can be found in #334.