RFR: 7904152: jtreg tests fail with openjdk 25

[Vladimir Petko]

The policy tests try to set the Security manager on the command-line causing the following error:

java.lang.Error: A command line option has attempted to allow or enable the Security Manager. Enabling a Security Manager is not supported.
       at java.lang.System.initPhase3([java.base at 25.0.2](mailto:java.base at 25.0.2)/System.java:1969)

The last supported JDK that allows setting the security manager is 21, so I have added `JDK21HOME` in line with other JDK-specific variables. 
The alternative is to name the variable JDK23HOME as the security manager was disabled in openjdk-24[1]

[1] https://openjdk.org/jeps/486

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Commit messages:
 - fix: add JDK21HOME for PolicyTest

Changes: https://git.openjdk.org/jtreg/pull/315/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=315&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904152
  Stats: 11 lines in 3 files changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/315.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/315/head:pull/315

PR: https://git.openjdk.org/jtreg/pull/315