docs: link the 3.x deprecation banner to the migration page

[nikagra]

Depends on: nothing (the scylla-4.19.2.x fast-forward is done, and /stable/upgrade_guide/from_3x/ is live)
Blocks: nothing

The deprecation banner on every 3.x page links to the upgrade guide's root, not to the dedicated 3.x → 4.x page #997 added. That was deliberate: /stable/ was built from a frozen release branch without from_3x/. Now that scylla-4.19.2.x only fast-forwards from scylla-4.x, the page reaches /stable/ with the next fast-forward.

• point the banner's "migration guide" link at /stable/upgrade_guide/from_3x/
• replace the template comment that said to repoint the link once the page reached /stable/

#1001 moves the 3.x manual's own links the same way.

Verified: a local four-version multiversion build with this conf.py, scylla-3.11.5.x = scylla-3.x + #1001, and scylla-4.19.2.x = scylla-4.x (its state after the fast-forward): exit 0, javadoc guard green. The banner on 3.x pages links to …/stable/upgrade_guide/from_3x/, and stable/upgrade_guide/from_3x/index.html exists in that build. The target page is live since 2026-09-30.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: QUIET

Plan: Advanced

Run ID: f5cd549e-5a71-4613-9bc2-8a4feeaed1df

📥 Commits

Reviewing files that changed from the base of the PR and between 4b3dbf6 and a7a4794.

📒 Files selected for processing (1)

• docs/source/_templates/version-warning.html

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📝 Walkthrough

Walkthrough

The deprecated Java Driver 3.x notice now links to /stable/upgrade_guide/from_3x/. The comment describes the link target and publishing-branch setup. The conditional and notice content are unchanged.

Priority: ⬇️ Low

Change: Other

Merge Risk: ⚪ Minimal · up to a7a47

The deprecated 3.x notice links to the published upgrade guide; no actionable merge risk remains.

Architecture Summary

Architecture risk: 🔵 Low · up to a7a47

The change affects 1 system.

Changed systems: docs

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — docs (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in docs/source/_templates/version-warning.html: The comment now describes the /stable/upgrade_guide/from_3x/ target and publishing-branch setup, replacing the prior note about linking to the guide root and when to repoint the link.
• observed — Modified behavior in docs/source/_templates/version-warning.html: The deprecated-version notice’s migration link now targets /stable/upgrade_guide/from_3x/ instead of /stable/upgrade_guide/.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: updating the 3.x deprecation banner to link to the migration page.
Description check	✅ Passed	The description directly explains the link update, its release-branch context, and verification results.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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