RFR: 7904048: Update jtreg to use Ant 1.10.15

[Christian Stein]

Please review this update to make jtreg use Ant 1.10.15

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Commit messages:
 - 7904048: Update jtreg to use Ant 1.10.15

Changes: https://git.openjdk.org/jtreg/pull/270/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=270&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904048
  Stats: 8 lines in 3 files changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jtreg/pull/270.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/270/head:pull/270

PR: https://git.openjdk.org/jtreg/pull/270