RFR: 7903005: jtreg plugin does not work with intellij 2021.2

[Maurizio Cimadamore]

This patch adds support for 2021.2.
I've bumped the dependency on the gradle IntelliJ plugin.
I've also tweked the way in which the supported IDE versions are set, by tweaking the `patchXML` task to set the correct `since` build.
Turns out, the previously generated plugin.xml had a IJ version hardcoded in it, due to bad gradle configuration.

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Commit messages:
 - Merge branch 'master' into 2021_2_support
 - Add support for 2021.2
 - Add logic for discovering all jars in the jtreg lib folder

Changes: https://git.openjdk.java.net/jtreg/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903005
  Stats: 8 lines in 2 files changed: 2 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/19/head:pull/19

PR: https://git.openjdk.java.net/jtreg/pull/19