change index backwards compatibility policy. [LUCENE-5940]

Currently, our index backwards compatibility is unmanageable. The length of time in which we must support old indexes is simply too long.

The index back compat works like this: everyone wants it, but there are frequently bugs, and when push comes to shove, its not a very sexy thing to work on/fix, so its hard to get any help.

Currently our back compat "promise" is just a broken promise, because we cannot actually guarantee it for these reasons.

I propose we scale back the length of time for which we must support old indexes.

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Migrated from [LUCENE-5940](https://issues.apache.org/jira/browse/LUCENE-5940) by Robert Muir (@rmuir), updated Dec 31 2020
Linked issues:
- #6555
- #6969
- #6996
- #7001
- #6441
- #6525
- #6189