Add an OpenVox 8 to 9 upgrade guide to the 9.x docs

[miharp]

Adds an upgrade-planning page to the 9.x collection, "Upgrading from OpenVox 8 to OpenVox 9", covering what to review before moving a production deployment, alongside the existing package-mechanics page (upgrade_minor). Modeled on the shape of Puppet Core's 8-to-9 guide, with every claim checked against the OpenVox 9 prerelease sources rather than copied.

Part of #456. Draft until #457, #458, and #459 merge: the page links to the server 9.x, OpenVoxDB 9.x, and OpenFact 6.x release notes and to the server 9.x auth.conf page, which those PRs add. Everything else on the page resolves on master today.

What the page covers

• Component table: Ruby 3.2 to 4.0, OpenSSL 3.0 to 3.5, OpenFact 5 to 6, JRuby 9.4 to 10.1, Java 21 or 25 (17 dropped), with a link to the component-versions page for exact versions.
• Before-you-upgrade checklist: latest 8.x first, per-component release notes, platform coverage (Debian 11/12 dropped for server and OpenVoxDB, Amazon Linux 2 for server), backups.
• Ruby 4.0 review guidance for custom facts, functions, types, providers, and agent- or server-installed gems, including the Kernel#open pipe removal.
• OpenFact 6 changes that affect fact code (Ruby 3.0+, exec/which deprecations, time_limit/limit aliases, ldapname removal, /opt/puppetlabs/bin search path).
• Behavior changes verified in the prerelease sources: deferred functions preprocessed by default again (openvox#462), report storage opt-in (openvox#583), and the server setting fallback (openvox#536). On the last one the page documents what the code does in beta2: root agents still fall back with a deprecation warning, non-root runs fail. That is more precise than the core 9.x install_pre page, which says the fallback is gone; worth aligning the two before GA.
• Removed settings (configprint, pluginsync, data_binding_terminus, environment_data_provider) and other removals (regsubst encoding argument, pe_serverversion, zone_core, Java keystores, legacy PAL APIs).
• Server and OpenVoxDB changes: Java 21/25, the filebucket read-authorization change (openvox-server#549), Jetty 12 with the OpenVoxDB bootstrap.cfg jetty10-service pitfall for upgrades from 8.14.0 or earlier, PostgreSQL 14 minimum, and the server package's dependency on agent 9.
• Test-then-upgrade checklist and upgrade order.

Also adds the nav entry (before "Upgrading OpenVox 9") and cross-links the page from the breaking-changes callout in upgrade_minor.

File	Change
upgrade_major.md	New page
upgrade_minor.md	Callout now links to the new page
openvox_9x.yml	Nav entry

Checks

• markdownlint clean; jekyll build clean; the page renders under /openvox/9.x/upgrade_major.html and the nav and upgrade_minor callout link to it.
• The only links that do not resolve on this branch are the four into the trees that Add OpenVox Server 9.x docs collection as a preview (latest stays on 8.x) #457, Add OpenVoxDB 9.x docs collection as a preview (latest stays on 8.x) #458, and Add OpenFact 6.x docs collection as a preview (latest stays on 5.x) #459 add.
• Screenshots to follow once the dependent PRs land and the cross-links resolve.