docs: publish scylla-4.x as the stable docs version (DRIVER-1036)

[nikagra]

BRANCHES in docs/source/conf.py already lists every release branch that exists upstream, so there is nothing left to register and /stable/ stays pinned to scylla-4.19.0.x — a branch sitting at 4.19.0.1-4-gf117e2b896, cut long before client routes landed. Everything merged to scylla-4.x since is invisible on the site, including the client-routes section in manual/core/address_resolution/.

• Add scylla-4.x to BRANCHES and make it LATEST_VERSION, so /stable/ is the development tip.
• Repoint the deprecation banner's migration link at upgrade_guide/from_3x/ — its comment deferred exactly this until /stable/ carried that page.
• TAGS stays empty, and scylla-4.x stays out of scylladb_markdown_recommonmark_versions, which is what selects MyST for its {eval-rst} fences.

This follows the sibling drivers rather than inventing a policy: gocql has LATEST_VERSION = "master", cpp-driver has BRANCHES = ['master'] and LATEST_VERSION = 'master', both with TAGS = []. The trade-off is that /stable/ now means "development tip", not "newest release branch".

Verified: make -C docs test green under -W --keep-going; sphinx-multiversion --dump-metadata lists 17 versions with scylla-4.x among them; multiversion_regex_builder(BRANCHES) matches scylla-4.x, and LATEST_VERSION is in BRANCHES as conf.py requires. Not covered: the full 17-version publish, which only runs post-merge.

Stacked on #1004 — Docs / Publish has been red on every run since 2026-05-26, so nothing reaches the site until that lands. scylla-4.x builds with <release>11</release>, which is why #1004's per-branch JDK map is the prerequisite here and a blanket JDK 8 is not.

Fixes DRIVER-1036

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