chore(renovate): track SQL Server CU/GDR tags on a fixed Ubuntu base

[samtrion]

Summary

Adds a Renovate package rule for mcr.microsoft.com/mssql/server that uses regex versioning, so only cumulative update (CU) and GDR tags on a fixed Ubuntu base get tracked.

Motivation

• SQL Server 2025 images on ubuntu-24.04 crash intermittently at startup ([2025] Intermittent sqlpal crash on startup , ubuntu-24.04 only (~1.7% of starts, ~8.3% under concurrency) microsoft/mssql-docker#974): about 1.7% of single starts and about 8.3% with 3 concurrent starts. In roughly 250 starts on ubuntu-22.04 there were no crashes. Downstream repositories therefore pin the ubuntu-22.04 variants.
• The generic docker rule uses "versioning": "loose". That cannot order SQL Server tags such as 2025-CU1-ubuntu-22.04, so a repository that started on a -preview tag never got a CU update proposed.
• test(integration): survive intermittent SQL Server 2025 startup crashes pulse#796 added this rule locally, but the template sync overwrites renovate.json, so the rule has to live here.

Change

• New packageRules entry, placed after the generic docker/loose rule so it takes precedence:
  • matchDatasources: ["docker"], matchPackageNames: ["mcr.microsoft.com/mssql/server"]
  • versioning: regex:^(?<major>\d+)-CU(?<minor>\d+)(-GDR(?<patch>\d+))?-ubuntu-(?<compatibility>\d+\.\d+)$
• The compatibility group keeps updates on the Ubuntu base that is already in use. Preview, RC, CTP and latest tags never match.

Validation

• renovate-config-validator (renovate 44.115.10) reports no new findings. The only warning (vulnerabilityAlerts.prPriority) was already on main.
• Checked the regex against sample tags: 2025-CU1-ubuntu-22.04 and 2022-CU21-GDR2-ubuntu-22.04 match; 2025-RC1-ubuntu-24.04, 2025-preview-ubuntu-24.04, 2022-CTP2.0-ubuntu-22.04 and 2025-latest do not.
• The rule is identical to the one in test(integration): survive intermittent SQL Server 2025 startup crashes pulse#796.