chore(release): bump version to 15.8.0

[marevol]

Summary

Prepares the 15.8.0 release: moves off the 15.8.0-SNAPSHOT parent, points the
build scripts at the matching fess-15.8 / fess-kopf v15.8.0 artifacts, and
updates the release-time values that are not derived from the project version
(EOL date, documentation links, and the orig/view JSP copies).

Changes Made

Version / build

• pom.xml: parent version 15.8.0-SNAPSHOT → 15.8.0
• dbflute.xml: branch.name fess-15.7 → fess-15.8 (mydbflute source branch)
• deps.xml: kopf.version 15.7.0 → 15.8.0 (fess-kopf release artifact)

EOL

• SystemHelper: EOL date 2027-12-01 → 2028-02-01

Documentation

• README.md: documentation links and download instructions 15.7 → 15.8,
  and drop the TAR.GZ package from the requirements list — Fess publishes DEB,
  RPM and ZIP only, which is what the same file already says two sections
  further down
• docs/{de,es,fr,ja,ko,pt-BR,zh-CN}/README.md: same links, which had been left
  behind at 15.6
• src/main/config/openapi/v2/openapi-user.yaml: externalDocs.url 15.7 →
  15.8 (info.version was already 15.8.0)

The online.help.* links need no change — SystemHelper substitutes
{version} from project.properties at runtime.

orig/view JSPs

AdminDesignAction.editAsUseDefault reads WEB-INF/orig/view/<file>, so a copy
that was not updated alongside its WEB-INF/view counterpart silently hands the
administrator an older page. Two had drifted and are now byte-identical again:

• chat/chat.jsp — feat(rag): notify browser of LLM retry/waiting/fallback/warning and search hit count #3130 added the retry / waiting-queue / hit-count / fallback
  / warning message wiring to the view copy only. This file is one of the 18
  registered in fess.xml, so the editor does serve it.
• error/redirect.jsp — fix(error): route genuine 400s to the request error page and keep 401s intact for API clients #3188 restructured the badAuth branch in the view copy
  only. Not registered in the design editor, but the pair is meant to stay
  identical.

Testing

• mvn formatter:format && mvn license:format — no files changed
• mvn test -Dtest=SystemHelperTest — 83 tests, 0 failures
• mvn test -Dtest=AdminDesignActionTest — 23 tests, 0 failures
• diff -r WEB-INF/orig/view WEB-INF/view — no differing file remains
• Verified the upstream artifacts this PR references exist: fess-parent:15.8.0
  on Maven Central, fess-kopf tag v15.8.0, lastaflute-example-waterfront
  branch fess-15.8
• Verified every rewritten documentation URL returns HTTP 200

Breaking Changes

• None

Additional Notes

• No functional Java changes.
• The EOL date follows the published policy on https://fess.codelibs.org/eol.html
  (~18 months after release; +2 months per minor, matching 15.6 → 2027-10-01 and
  15.7 → 2027-12-01).
• Follow-up outside this repository: the maintenance table on the EOL page does
  not list 15.8.x yet.