RFR: 8301971: Make JDK source code UTF-8 [v3]

[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 incrementally with three additional commits since the last revision:

 - Also document UTF-8 requirements (solves JDK-8338973)
 - Let configure only accept utf-8 locales
 - Address review comments from Kim

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24574/files
  - new: https://git.openjdk.org/jdk/pull/24574/files/38004164..452f42dc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24574&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24574&range=01-02

  Stats: 47 lines in 7 files changed: 27 ins; 2 del; 18 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