RFR: 8299852: Modernize ConcurrentHashMap [v3]

[Per Minborg]

> `java.util.concurrent.ConcurrentHashMap` is relatively old and has not been updated to reflect the current state of Java and can be modernized: 
> 
> * Add `@Serial` annotations 
> * Seal classes and restrict subclassing for internal classes 
> * Use pattern matching for instance 
> * Remove redundant modifiers (such as some final declarations) 
> * Use Objects.requireNonNull for invariant checking 
> * Use diamond operators instead of explicit types 
> * Simplify expressions using Math::max

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge master
 - Add @Override annotations
 - Untrack file
 - Modernize ConcurrentHashMap

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Changes: https://git.openjdk.org/jdk/pull/11924/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11924&range=02
  Stats: 534 lines in 1 file changed: 164 ins; 21 del; 349 mod
  Patch: https://git.openjdk.org/jdk/pull/11924.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11924/head:pull/11924

PR: https://git.openjdk.org/jdk/pull/11924