RFR: 8357436: Change jspawnhelper warning recommendation from VFORK to FORK

[Aleksey Shipilev]

In [JDK-8352533](https://bugs.openjdk.org/browse/JDK-8352533), we print the suggestion to use VFORK in jspawnhelper misbehaves. But [JDK-8357179](https://bugs.openjdk.org/browse/JDK-8357179) deprecated VFORK! So the warning message should actually suggest using FORK. This seems to be in line with [JDK-8357180](https://bugs.openjdk.org/browse/JDK-8357180) CSR discussion and the pending [JDK-8357181](https://bugs.openjdk.org/browse/JDK-8357181) release note.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `java/lang/Process java/lang/ProcessBuilder`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25344/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357436
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25344.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25344/head:pull/25344

PR: https://git.openjdk.org/jdk/pull/25344