Check the minimum Java version in build system and describe it in docs
The issue was found by @marcin during 1.15 sanity checks: #1296 (comment 434164)
The current version of the OpenAPI generator requires at least OpenJRE 11. The build system checks only if the java
binary is present and the documentation specifies that Java is required without providing a minimum version.
We should somehow cover the java requirement checks in the rake files or/and the docs.