CI: refresh ClickHouse version pins — roll EOL 25.3 LTS → 26.3 (go-ahead requested per AGENTS.md CI policy)

[polyglotAI-bot]

Summary

The ClickHouse version pins in this repo's CI workflows have gone stale relative to the current support window, and the older pinned LTS (25.3) is now effectively EOL. This proposes rolling the pins forward.

Per AGENTS.md — "Do not modify CI workflow files (.github/workflows/**). CI changes are restricted; raise it with the user first." — I'm raising this first and requesting a maintainer 👍 before opening a PR. There's one small judgment call (below) I'd like confirmed.

Background

• ClickHouse 26.7 is out; 26.4 and 25.3 are outside the current support window.
• This repo's CI model is 2 LTS + latest (not an explicit N-latest matrix). latest already exercises 26.7, so the applicable change is refreshing the two pinned LTS versions, not adding a 26.7 pin.
• The current LTS set is 25.8 and 26.3.

Current state (origin/main)

• .github/workflows/build.yml
  • L35 PREFERRED_LTS_VERSION: "25.3"
  • L154 / L306 / L366 clickhouse: ["25.3", "25.8", "latest"] (three jobs)
• .github/workflows/analysis.yml — L31 PREFERRED_LTS_VERSION: "25.3"
• .github/workflows/nightly.yml — L16 CH_VERSION: "25.3"
• .github/workflows/release.yml — L7 CH_VERSION: "25.3"

(build.yml L215 ["cloud"], run_examples.yml default 25.8, and test_head.yml head are intentionally left unchanged.)

Proposed change

• Matrix (×3 in build.yml): ["25.3", "25.8", "latest"] -> ["25.8", "26.3", "latest"] — drop EOL 25.3, add current LTS 26.3, keep 25.8 + latest.
• Single-version env vars (PREFERRED_LTS_VERSION ×2, CH_VERSION ×2): 25.3 -> 25.8.

The clickhouse/clickhouse-server:25.8 and :26.3 image tags both exist. CI-only change: no client code / tests / docs, so no CHANGELOG entry.

The one judgment call: what should the single-version env vars become?

PREFERRED_LTS_VERSION / CH_VERSION feed the non-matrix jobs (analysis, nightly, release) with one representative LTS. I propose 25.8 (the more battle-tested of the two pinned LTS), based on this repo's own history — these vars have consistently tracked the older of the two pinned LTS:

Matrix LTS pair	PREFERRED_LTS_VERSION / CH_VERSION
24.3, 24.8	24.3
25.3, 25.8	25.3
25.8, 26.3 (proposed)	25.8

(Commit 76c335d0 unified PREFERRED_LTS_VERSION and CH_VERSION onto the older LTS 25.3.)

Alternative: if you'd prefer these track the newest LTS, they'd become 26.3 instead. Happy either way — just confirm.

Ask

1. 👍 to make this CI change (per the AGENTS.md CI-change policy).
2. Confirm the env-var value: 25.8 (my recommendation) or 26.3.

Once confirmed I'll open the PR (polyglot/ci-ch-lts-refresh -> main).