[jdk21u-dev] RFR: 8318082: ConcurrentModificationException from IndexWriter

[Aleksey Shipilev]

Unclean backport to fix a customer-visible bug in Javadoc tool. The uncleanliness comes from the missing refactoring ([JDK-8309566](https://bugs.openjdk.org/browse/JDK-8309566), that looks unnecessary to backport.

Maybe @jonathan-gibbons and @pavelrappo want to take a look.

Additional testing:
 - [x] New test fails without the product fix, passes with it
 - [x] `jdk/javadoc` still passes

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Commit messages:
 - Backport fc29a2e152310ed81bd1bb23e6f17d02f055a454

Changes: https://git.openjdk.org/jdk21u-dev/pull/23/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=23&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318082
  Stats: 240 lines in 9 files changed: 205 ins; 28 del; 7 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/23.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/23/head:pull/23

PR: https://git.openjdk.org/jdk21u-dev/pull/23