RFR: 8356644: Update encoding declaration to UTF-8

[Magnus Ihse Bursie]

A handful of html and xml files in the JDK source tree claims to have encodings like `ISO-8859-1`, when they are in fact pure US-ASCII files.

While perhaps technically correct, this is misleading, and goes contrary to the efforts of turning the source code into UTF-8 proper.

I chose between marking them as "ASCII" and "UTF-8", but chose the latter, since otherwise if they ever were to be updated with a non-ASCII character, the value would have been unspecified, and after JDK-8301971, all files in the JDK repository will be interpreted as UTF-8.

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Commit messages:
 - 8356644: Update encoding declaration to UTF-8

Changes: https://git.openjdk.org/jdk/pull/25148/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25148&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356644
  Stats: 794 lines in 53 files changed: 2 ins; 9 del; 783 mod
  Patch: https://git.openjdk.org/jdk/pull/25148.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25148/head:pull/25148

PR: https://git.openjdk.org/jdk/pull/25148