<i18n dev> RFR: 8309748: Improve host selection in `External Specifications` page

[Hannes Wallnöfer]

Please review a change to improve selection by host name in the External Specifications page. After trying a list of checkboxes as used in the API summary pages and finding it not a suitable solution, I found that a select/dropdown menu works best for this page. 

 - [New external specs page](https://cr.openjdk.org/~hannesw/8309748/api.00/external-specs.html)
 - [Old external specs page](https://docs.oracle.com/en/java/javase/25/docs/api/external-specs.html) for comparsion

I also updated `@spec` tags in `java.lang.Character` to point to `www.unicode.org` rather than `unicode.org` to avoid a  `unicode.org` entry in the host list (other `@spec` tags use the `www.unicode.org` host name). The short `unicode.org` URL is still used elsewhere in the `java.lang.Character` class, I can update those links to the canonical host name as well if desired.

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Commit messages:
 - 8309748: Improve host selection in `External Specifications` page

Changes: https://git.openjdk.org/jdk/pull/28863/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28863&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309748
  Stats: 97 lines in 5 files changed: 71 ins; 3 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/28863.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28863/head:pull/28863

PR: https://git.openjdk.org/jdk/pull/28863