RFR: 8299852: Modernize ConcurrentHashMap

[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

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Commit messages:
 - Untrack file
 - Modernize ConcurrentHashMap

Changes: https://git.openjdk.org/jdk/pull/11924/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11924&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299852
  Stats: 385 lines in 1 file changed: 14 ins; 22 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