RFR: 8316693: Simplify at-requires checkDockerSupport()

[Mikhailo Seledtsov]

Please review this simple change.

Currently JTReg @requires extension test/jtreg-ext/requires/VMProps.java checkDockerSupport() uses "<CONTAINER_ENGINE> ps" command to check whether test host/environment allows for container testing. With this change proposing to switch it to "which <CONTAINER_ENGINE>":
  - the "ps" command could be heavy-weight in some environments, taking longer time then expected
  - it may not apply to all container engine types

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Commit messages:
 - 8316693: Simplify at-requires checkDockerSupport()

Changes: https://git.openjdk.org/jdk/pull/16111/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316693
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16111.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16111/head:pull/16111

PR: https://git.openjdk.org/jdk/pull/16111