RFR: 7903418: It is not possible to build jtreg 6.1 branch using jdk 11.0.18

Fri Jan 27 03:22:22 UTC 2023

It is not possible to build the jtreg 6.X using the recently released JDK 11.0.18. The root cause is a grep command in the Defs.gmk which checks the usage of jdk8. Because of that, all GitHub actions for jdk17u-dev and jdk11u-dev are broken:
see https://github.com/mrserb/jdk11u-dev/actions/runs/4018535809/jobs/6904278681

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Commit messages:
 - Improve support of "legacy platforms"

Changes: https://git.openjdk.org/jtreg/pull/146/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=146&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903418
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/146.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/146/head:pull/146

PR: https://git.openjdk.org/jtreg/pull/146