RFR: 8357993: Use "stdin.encoding" for reading System.in with InputStreamReader/Scanner [hotspot]

[Volkan Yazici]

Passes the `Charset` read from the `stdin.encoding` system property while creating `InputStreamReader` or `Scanner` instances for `System.in`.

`stdin.encoding` is a recently added property for Java 25 in [JDK-8350703](https://bugs.openjdk.org/browse/JDK-8350703). Employing it throughout the entire code base is addressed by the parent ticket [JDK-8356893](https://bugs.openjdk.org/browse/JDK-8356893). JDK-8357993 this PR is addressing is a sub-task of JDK-8356893 and is concerned with only areas related to Hotspot.

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Commit messages:
 - Remove redundant assert
 - Discard changes unrelated with build and tools
 - Revert superfluous changes
 - Revert changes to 3rd parties in `com/sun/org/apache`
 - Revert `PandocFilter` changes
 - Use `stdin.encoding` in `InputStreamReader` and `Scanner` instantiations

Changes: https://git.openjdk.org/jdk/pull/25542/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25542&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357993
  Stats: 13 lines in 4 files changed: 4 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/25542.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25542/head:pull/25542

PR: https://git.openjdk.org/jdk/pull/25542