RFR: 8300866: Declare some classes final in java.io [v2]

[Per Minborg]

> This PR proposes declaring some internal classes in java.io as `final`, explicitly indicating that they are not intended for sub-classing. 
> 
> If you think declaring classes final is just "static noice", please see http://minborgsjavapot.blogspot.com/2021/12/why-general-inheritance-is-flawed-and.html first and then come back and comment on the issue.

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

 - Merge master
 - Declare classes as final

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Changes: https://git.openjdk.org/jdk/pull/12141/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12141&range=01
  Stats: 15 lines in 5 files changed: 1 ins; 3 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12141.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12141/head:pull/12141

PR: https://git.openjdk.org/jdk/pull/12141