RFR: 7904152: jtreg tests fail with openjdk 25 [v2]

[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

Vladimir Petko has updated the pull request incrementally with one additional commit since the last revision:

  Update doc/building.md
  
  Co-authored-by: Christian Stein <sormuras at gmail.com>

-------------

Changes:
  - all: https://git.openjdk.org/jtreg/pull/315/files
  - new: https://git.openjdk.org/jtreg/pull/315/files/54715cfe..cd2772d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=315&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=315&range=00-01

  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 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