RFR: 8331941: Make CollectedHeap::parallel_object_iterator public

[Albert Mingkun Yang]

Simple (or even trivial) change of making an api public so that one can drop the `friend` declaration.

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

Commit messages:
 - par-obj-iter

Changes: https://git.openjdk.org/jdk/pull/19141/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331941
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19141.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19141/head:pull/19141

PR: https://git.openjdk.org/jdk/pull/19141