[#1128] Replace the DocBook xinclude and olink leftovers of the generated reference with AsciiDoc

[vharseko]

Fixes #1128

The generated reference still carried DocBook markup from before the AsciiDoc migration; Asciidoctor printed it as text, and the link checker of doc.openidentityplatform.org reported the href targets as broken.

Where	Occurrences	Was	Now
dsconfig property, "Allowed Values" of a duration	244	<xinclude:include href="itemizedlist-duration.xml" />	inline text naming the units (ms, s, m, h, d, w)
dsconfig property, "Allowed Values" of an ACI (global-aci) — not listed in the issue, same kind of leftover	2	<olink targetdoc="admin-guide" targetptr="about-acis" />	xref:../admin-guide/chap-privileges-acis.adoc#about-acis[…], in the form admin-tools-ref.adoc already uses
upgrade description	1	<xinclude:include href="description-upgrade.xml" />	include::./_description-upgrade.adoc[] (new partial)
backendstat show-index-status	1	<xinclude:include href="variablelist-backendstat-index-status.xml" />	include::./_variablelist-backendstat-index-status.adoc[] (new partial)

• The duration and ACI values sit inside the "Allowed Values" item of a description list in an open block, where an included list would break the markup 244 times and an xref to another page degrades to plain text in the man pages; the duration syntax is therefore written inline. The message keys drop their _REL_URL suffix (REF_DSCFG_ACI_SYNTAX, REF_DSCFG_DURATION_SYNTAX), ordinals unchanged; there are no localized copies.
• The upgrade and backendstat supplements follow the pattern already used for _description-rate-tools.adoc and _description-psearch-info.adoc. The backendstat partial describes the current output rather than the 3.x DocBook one: the subcommand is show-index-status, and the columns are Raw DB Name, Valid, Confidential, Over Entry Limit.
• visitDuration also glued its sentences together ("Lower limit is 0 seconds.Upper limit is …", 85+ times on the site); they are now separated by a space.

Verification

• Generated man-dsconfig.adoc, man-upgrade.adoc and man-backendstat.adoc the way GenerateRefEntriesMojo does (gendoc --help on the packaged server classpath). Before: 244 duration + 2 olink leftovers in dsconfig, matching the published page, and one each in upgrade and backendstat. After: none.
• Rendered the upgrade and backendstat pages and two dsconfig subcommands with AsciidoctorJ 2.5.3, html5 and manpage backends: both includes resolve, no warnings, the xref becomes ../admin-guide/chap-privileges-acis.html#about-acis.
• The full Antora site build was not run locally.