Jira upgrade reindexing docs misleading

Documentation Suggestion

Current State

The "Upgrade Jira" documentation misleads users that a reindex is always required.

The following documents:

•  https://confluence.atlassian.com/adminjiraserver/upgrading-jira-installer-938846937.html
• https://confluence.atlassian.com/adminjiraserver/upgrading-jira-manual-938846939.html#UpgradingJira(manual)-post-upgrade

state that:

Since your old index is incompatible, reindex Jira to rebuild it. This step might take some time, depending on how many issues and apps you have.

And the following documents:

• https://confluence.atlassian.com/adminjiraserver/upgrading-jira-data-center-installer-966063332.html#UpgradingJiraDataCenter(installer)-post-upgrade
• https://confluence.atlassian.com/adminjiraserver/upgrading-jira-data-center-manual-938846951.html

Include a Step 4 which says:

Step 4. Rebuild index
Reindex Jira to recreate your index. This step might take some time, depending on how many issues and apps you have.

Why this needs to be improved

The 4 documents linked above suggest that the index should, or must, be rebuilt after an upgrade.

In fact, a reindex is only required when upgrading past Major versions (Jira 7 -> 8, for example), not for minor or bugfix versions (Jira 9.4 -> Jira 9.12)

The current formulation in the documentation is problematic because:

• Waiting for a superfluous index to complete introduces an unnecessary time delay during upgrades, protracting the customer's upgrade window.
• It introduces an unnecessary risk during the upgrade window:
  • The reindex may fail, which could cause complications for the admin.
  • Separately, if some plugins happen to not be enabled at the reindex time, those fields will have their index unnecessarily erased
• The ZDU documentation links directly to these articles. A reindex should not absolutely not be performed during ZDU. A design goal of ZDU is that a manual reindex is not needed to be performed, so that nodes can keep their own discreet Jira version and index while the cluster is in upgrade mode

We have non-0 count of anecdote in customer environment of these items causing problems. Thus, we should update the documentation to reflect more accurately when a reindex is required

Suggested solution

1. Update the 4 articles above to include that reindexing should only be performed for major version upgrades
2. Add a statement to the ZDU documentation section "Upgrading Jira" that says to skip the instructions in the child page about reindexing