Update instructions for index upgrade in SolrCloud mode

[rahulgoswami]

https://issues.apache.org/jira/browse/SOLR-XXXXX

Description

UPGRADEINDEX CoreAdmin API briefly mentions a way to achieve the same goal (of index upgrade) in SolrCloud mode. This PR elaborates on an optimized way to do the same so that progress is preserved across restarts, without blocking external writes, just like with the API.

[rahulgoswami]

@dsmiley Would appreciate a review here. If this looks good, would also like to cherry-pick to branch_9_11. Thanks!

[rahulgoswami]

coreadmin-api.html

Attaching the corresponding asciidoctor generated html for reference

[dsmiley]

Looks good; thanks.

[rahulgoswami]

Thanks @dsmiley. Do you think a note about the UPGRADEINDEX API also belongs in https://github.com/apache/solr/blob/main/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc ?

[dsmiley]

No because it's not specific to the upgrade to Solr 9.whatever.

upgrading-a-solr-cluster.adoc is linked from the parent of these per-version page. It's title suggests it's absolutely the place for referring to this information. It's current content, maybe not, but it's scope could be expanded.

[rahulgoswami]

upgrading-a-solr-cluster.adoc is linked from the parent of these per-version page. It's title suggests it's absolutely the place for referring to this information. It's current content, maybe not, but it's scope could be expanded.

I agree. I also see some outdated/incomplete information wrt Lucene's IndexUpgraderTool there. Will try to revisit this. Thanks.