RFR: 7903503: Add Gradle legal file to jtreg

[Christian Stein]

Add Gradle legal file to jtreg as `legal/gradle.md` into `plugins/idea/gradle` directory; similar to https://github.com/openjdk/jfx/blob/master/gradle/legal/gradle.md used in OpenJFX.

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Commit messages:
 - Add Gradle legal file to jtreg

Changes: https://git.openjdk.org/jtreg/pull/160/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=160&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903503
  Stats: 231 lines in 1 file changed: 231 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/160.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/160/head:pull/160

PR: https://git.openjdk.org/jtreg/pull/160