RFR: 8329012: IGV: Update required JDK version in README.md

[Jie Fu]

While building IGV, I found the jdk version described in the README.md is incorrect.

According to the pom.xml: https://github.com/openjdk/jdk/blob/master/src/utils/IdealGraphVisualizer/pom.xml#L82 
It should be updated to "between 17 and 21".

Thanks.
Best regards,
Jie

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Commit messages:
 - 8329012: IGV: Update required JDK version in README.md

Changes: https://git.openjdk.org/jdk/pull/18481/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18481&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329012
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18481.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18481/head:pull/18481

PR: https://git.openjdk.org/jdk/pull/18481