RFR: 7902912: Update IntelliJ Plugin build Readme [v2]

[Thomas Stuefe]

> Hi, may I have reviews for this small documentation fix please? I am not a CODETOOLS committer so I may need someone to sponsor this fix.
> 
> Since I just built the intellij plugin in 2020.3, I stumbled about a couple of ambiguous infos which are not immediately clear to outsiders.
> 
> -> some UI features changed in Intellij
> -> Clarified the meaning of JTREG_HOME
> -> One thing I was not sure about was the required SDK level to build the plugin. The documentation says "Any JDK 8 is fine" which I took as "JDK8 is required" - proceeded to download JDK 8 but then I was unable to choose this JDK as a SDK for the IDEA JDK. It was, however, happy with a JDK 15 I gave it, and proceeded to build the plugin without any issues. Therefore I changed the sentence to "Any JDK 8++ is a fine choice".

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  Update plugins/idea/README.md
  
  Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

-------------

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/8/files
  - new: https://git.openjdk.java.net/jtreg/pull/8/files/eda907c8..494ed425

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=8&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=8&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/8/head:pull/8

PR: https://git.openjdk.java.net/jtreg/pull/8