[jdk11u-dev] RFR: 8301397: [11u, 17u] Bump jtreg to fix issue with build JDK 11.0.18

[Christoph Langer]

Bump GHA jtreg to 6.1+3 to unbreak GHA.

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Commit messages:
 - Bump jtreg version

Changes: https://git.openjdk.org/jdk11u-dev/pull/1700/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1700&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301397
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1700.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1700/head:pull/1700

PR: https://git.openjdk.org/jdk11u-dev/pull/1700