RFR: CODETOOLS-7903006: Upgrade jtreg plugin to IntelliJ 2021.2

[Thomas Stuefe]

Hi,

could we update target platform version to 21.2?

There were no issues with the build, plugin works fine (IntelliJ 2021.2 on Ubuntu x64).

Thanks, Thomas

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

Commit messages:
 - start

Changes: https://git.openjdk.java.net/jtreg/pull/20/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=20&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903006
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/20/head:pull/20

PR: https://git.openjdk.java.net/jtreg/pull/20