[jdk21u-dev] RFR: 8340969: jdk/jfr/startupargs/TestStartDuration.java should be marked as flagless

[SendaoYan]

Hi all,

This pull request contains a backport of commit [470701f0](https://github.com/openjdk/jdk/commit/470701f0bb269834cc0e1cb40f7d34e92226454b) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Leonid Mesnik on 6 Dec 2024 and was reviewed by SendaoYan and Erik Gahlin.

Thanks!

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Commit messages:
 - Backport 470701f0bb269834cc0e1cb40f7d34e92226454b

Changes: https://git.openjdk.org/jdk21u-dev/pull/1208/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1208&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340969
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1208.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1208/head:pull/1208

PR: https://git.openjdk.org/jdk21u-dev/pull/1208