SOLR-16995: Use leaderEligible property where it makes sense

[pvcnt]

https://issues.apache.org/jira/browse/SOLR-16995

Description

A new leaderEligible property was introduced on the Replica.Type in Solr 9 (?), but it is currently not used much in the codebase. Most of the code still uses something like replicaType == Type.NRT || replicaType == Type.TLOG, which makes the intention behind the code more difficult to read.

Solution

This PR is a refactoring using the leaderEligible property wherever it makes sense, to make the code more readable.

Tests

No new tests, as this refactoring does not introducing any new classes or methods, and does not change the existing behaviour.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[pvcnt]

Related to #1928 (split this particular change into a separate PR).

[dsmiley]

Nice.

BTW a JIRA isn't necessary for this low-impact internal refactoring. Just an FYI; since you created it, keep it.

[pvcnt]

@dsmiley It's actually the same JIRA as #1928. It just split my end goal (making it easier to work with replica types and possibly add new ones) into several PRs.

[pvcnt]

Failing tests are passing locally, they ma be flappers.

[HoustonPutman]

I love this. To many single-class/enum cases across the codebase, which makes adding new options much harder.