Remove legacy Universal Analytics snippet from built-in themes

[shenxianpeng]

Google shut down Universal Analytics in 2023, so the analytics.js snippet emitted by the built-in themes when the long-deprecated google_analytics config option was set has been sending visitor data to a dead service. This removes the snippet from both the mkdocs and readthedocs themes.

The google_analytics option itself remains accepted so that existing configs and third-party themes that read it keep working, but its deprecation warning now states that it has no effect on the built-in themes:

WARNING - Config value 'google_analytics': The configuration option google_analytics is
          deprecated and no longer has any effect: Google shut down Universal Analytics,
          and the built-in themes no longer include the legacy 'analytics.js' snippet.
          Use your theme's 'analytics' options (e.g. 'analytics.gtag' for Google
          Analytics 4) or override the 'analytics' template block instead.

Builds with --strict fail on the warning, as before. The modern theme.analytics.gtag (Google Analytics 4) option is unchanged.

Also adds a documentation example under "Overriding Template Blocks" showing how to plug in any analytics service by overriding the analytics block — the approach the upstream issue recommends.

Verified by building sites with both themes: with google_analytics set, output contains no analytics scripts and the warning above is emitted; with analytics.gtag set, the gtag snippet is still rendered. Build/theme/config test suites pass (394 tests).

Related Issue

Addresses upstream mkdocs/mkdocs#3630 (also related upstream: mkdocs/mkdocs#1794, mkdocs/mkdocs#2101).

Checklist

• New tests added for new behavior (if applicable) — removal only; existing suites cover the themes and config
• Documentation updated (if applicable)
• Release notes docs/about/release-notes.md updated (if applicable)

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https://claude.ai/code/session_018Hw7x9BxxpuKiLkzjLczp9

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