RFR: 8299852: Modernize ConcurrentHashMap [v2]

[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 incrementally with one additional commit since the last revision:

  Add @Override annotations

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11924/files
  - new: https://git.openjdk.org/jdk/pull/11924/files/feafe91b..91c4862b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11924&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11924&range=00-01

  Stats: 151 lines in 1 file changed: 150 ins; 0 del; 1 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