Online reindexing from previous versions is allowed in 3.12

Despite the Lucene version bump, online reindexing remains supported
during upgrades.

Lucene only guarantees backward compatibility for index formats of
the current and previous (N-1) major versions.

As a result:
- Gerrit 3.10/3.11 (Lucene 9.x) -> 3.12 (Lucene 10.x) works
- Gerrit 3.9 (Lucene 8.x) -> 3.12 (Lucene 10.x) does not work

Change-Id: I89f68cddb8afdbcee820ddf1e46a628777564ae8
1 file changed