Clarify Java compatibility requirements

[mltsy]

Description

Update Java compatibility deprecation message to clarify how to get the necessary JRE.

For those less familiar with the Java ecosystem, it's not clear that you can't get modern JREs from Java.com:

• https://stackoverflow.com/questions/72438409/firebase-tools-no-longer-supports-java-version-before-11-please-upgrade-to-java
• https://stackoverflow.com/questions/79105227/why-does-java-com-still-recommend-java-8-when-there-are-multiple-newer-lte-rele?noredirect=1#comment139487042_79105227

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[joehan]

Thanks for contributing this! I think this is a good clarification, and I'll check about getting the docs updated too