You can install either “JDK-15.0.1.jdk“ or “adoptopenjdk-15.jdk” directly from the Internet. However, LO won’t recognize them.
Did you read the bug report? The bug is exactly about not working (though recognizing) JDKs besides version 12.0.2 for whose installation the answer above provides a solution.
> The final string of Terminal commands is "LibreOffice -> Preferences -> LibreOffice > Advanced". At least in my case, the Terminal responds with "-bash: LibreOffice: command not found ".
This is not to be executed in “Terminal” but the menu path to LibreOffice configuration, where to check/set Java settings. This step is following step Start LibreOffice, hence not to be executed in “Terminal”. It is a common nomenclature to use “
menu 1 -> menu 2 -> ...” to indiciate: Select
menu 1 from menu bar, followed by (sub-)
menu 2 and so on …