RFR: 8301971: Make JDK source code UTF-8 [v6]

[Magnus Ihse Bursie]

> Most of the JDK code base has been transitioned to UTF-8, but not all. This has recently become an acute problem, since our mixing of iso-8859-1 and utf-8 in properties files confused the version of `sed` that is shipped with the new macOS 15.4.
> 
> The fix is basically simple, and includes the following steps:
> * Look through the code base for text files containing non-ASCII characters, and convert them to UTF-8, if they are not already
> * Update tooling used in building to recognize the fact that files are now in UTF-8 and treat them accordingly (basically, updating compiler flags, git attributes, etc).

Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - Merge branch 'master' into go-full-utf8
 - Merge branch 'master' into go-full-utf8
 - Add informative message  about supported locales.
 - Allow C locale with warning instead of fatal error
 - Also document UTF-8 requirements (solves JDK-8338973)
 - Let configure only accept utf-8 locales
 - Address review comments from Kim
 - Also tell javadoc that we have utf-8 now
 - Fix flags for Windows
 - Mark java and native source code as utf-8
 - ... and 4 more: https://git.openjdk.org/jdk/compare/2c1eb339...e68305db

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Changes: https://git.openjdk.org/jdk/pull/24574/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24574&range=05
  Stats: 177 lines in 13 files changed: 49 ins; 105 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/24574.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24574/head:pull/24574

PR: https://git.openjdk.org/jdk/pull/24574