RFR: 8275512: Upgrade required version of jtreg to 6.1

[Weijun Wang]

As a follow up of JEP 411, we will soon disallow security manager by default. jtreg 6.1 does not set its own security manager if JDK version is >= 18.

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Commit messages:
 - 8275512: Upgrade required version of jtreg to 6.1

Changes: https://git.openjdk.java.net/jdk/pull/6012/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6012&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275512
  Stats: 7 lines in 6 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6012.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6012/head:pull/6012

PR: https://git.openjdk.java.net/jdk/pull/6012