The version of the java executable should actually be checked

* [This code](https://github.com/antlr/antlr4-tools/blob/8d076f435728a56ffce18e972061d961a8ff14cf/antlr4_tool_runner.py#L91) "tests" the version of java, but it's false, so it never gets checked.
* Over [here in Antlr4BuildTasks](https://github.com/kaby76/Antlr4BuildTasks/issues/41), I don't check the version of java, but I need to because some versions of java don't work, e.g., version 7. Even a quick "java --version" would have found that v7 of java doesn't work because there's no "--version" option.
* The Antlr tool jar has a minimum requirement of version 11 or newer.
* The "Quick Start" instructions over on [antlr.org](https://www.antlr.org/) should stop defining the alias "alias antlr4='java -jar /usr/local/lib/antlr-4.10.1-complete.jar'" and just point to set up the tools from this repo. The tool here is much better because it could check the version of java, and download a version that can work if one tries to use the wrong version.

Start with antlr4_tool_runner.py at the version-checking code linked in the issue, then review the related Antlr4BuildTasks issue and the Quick Start instructions on antlr.org. Done should include an effective Java-version check, handling the stated minimum requirement, and updated Quick Start guidance pointing users to this repository's tools.

Written by the indexing model from the issue text.