[jdk20u] RFR: 8304878: ConcurrentModificationException in javadoc tool

[Ravi Reddy]

This is a backport of fix for JDK-8304878 to jdk20u from openjdk/jdk. Fix applies cleanly.
It fixes ConcurrentModificationException in javadoc tool.

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Commit messages:
 - 8304878: ConcurrentModificationException in javadoc tool

Changes: https://git.openjdk.org/jdk20u/pull/85/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20u&pr=85&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304878
  Stats: 77 lines in 2 files changed: 76 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk20u/pull/85.diff
  Fetch: git fetch https://git.openjdk.org/jdk20u.git pull/85/head:pull/85

PR: https://git.openjdk.org/jdk20u/pull/85