SOLR-17466: Inform 9x users about upcoming change to solrcloud default mode

[epugh]

https://issues.apache.org/jira/browse/SOLR-17466

Description

Let 9x users know about bin/solr start being cloud mode by default and that they need --standalone in 10.

Solution

Warnings

Tests

bats

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, not available for branches on forks living under an organisation)
• 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

[epugh]

Thanks @janhoy that is a great improvement....

[patrbraun]

Working for me. Could maybe use a line return after to increase visibility.

[epugh]

Working for me. Could maybe use a line return after to increase visibility.

Done!

[epugh]

I don't think users care about a warning message so skipping CHANGES.txt