Use better JDK names in unsupported version warning

[tyuldashev]

Description
In warning about unsupported JDK version currently some name of constant or something like that is used, e.g. JDK_1_8, JDK_11. It would be more user friendly if their actual version names are used, like 1.8 or 11

To Reproduce

1. Install UnitTestBot plugin built from main in IntelliJ IDEA
2. Open some project where Project or current module JDK is set to 21 (latest LTS version).
3. Invoke test generation dialog.

Actual behavior
Dialog with warning has been shown.

Expected behavior
Text in the warning would be something like:
JDK version 21 is not supported, use 1.8, 11 or 17 versions instead. Setup JDK.

Note I've also suggest to use JDK instead of SDK to be more specific if it's Java only warning.