Validate and update links (STF-557)

[oschwald]

Adds a lychee link-checker configuration and a
Links GitHub Actions workflow (push / pull_request / weekly schedule /
manual), then validates and updates the stale and redirecting links found
across the repo's Markdown and Go sources.

Link updates

• dev.maxmind.com/geoip/updating-databases?lang=en -> .../updating-databases/?lang=en (README.md x2, config_test.go x3, GeoIP.conf.default)
• dev.maxmind.com/geoip/upgrading-geoip-update?lang=en -> .../upgrading-geoip-update/?lang=en (README.md)
• dev.maxmind.com/geoip/updating-databases#3-run-geoip-update -> .../updating-databases/#3-run-geoip-update (doc/geoipupdate.md)
• golang.org -> go.dev (README.md)
• www.maxmind.com/en/my_license_key (404) -> www.maxmind.com/en/accounts/current/license-key (config_test.go x3, GeoIP.conf.default)
• www.maxmind.com/en/geoip2-services-and-databases -> www.maxmind.com/en/solutions/ip-geolocation-databases-api-services (doc/geoipupdate.md)
• support.maxmind.com/hc/en-us/articles/4408216129947-Download-and-Update-Databases#... -> support.maxmind.com/knowledge-base/articles/download-and-update-maxmind-databases (doc/geoipupdate.md)

The conf/GeoIP.conf.default source and the config_test.go fixtures that
mirror it were updated together to stay in sync.

False positives excluded in lychee.toml

• github.com/settings/ — redirects anonymous requests to the login flow
• http://test and http://Garbage:1080 — placeholder proxy/host values in config parser tests

max_redirects = 0 and 500..=599 in accept are intentional (matches the
dev-site/blog-site configs). Historical CHANGELOG.md entries are excluded from
scanning and left unchanged.

lychee summary after fixes: 19 OK, 0 Errors, 28 Excluded. Repo precious
linter (golangci-lint + prettier) passes and go test ./internal/geoipupdate/
is green.

Part of STF-557.

🤖 Generated with Claude Code

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)

[gemini-code-assist]

Code Review

This pull request introduces a Lychee link checker configuration file (lychee.toml), adds .lycheecache to .gitignore, and updates various outdated or redirected URLs across documentation, configuration templates, and test files. The reviewer suggested a useful improvement to the regular expression in lychee.toml for excluding GitHub blob URLs with line numbers, pointing out that the current pattern only matches hexadecimal commit SHAs and should be broadened to support branch names and tags.

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[gemini-code-assist]

The regular expression for excluding GitHub blob URLs with line numbers uses [0-9a-fA-F]+ to match the git reference (commit SHA). However, this will not match branch names (like main, master, dev) or tags (like v1.0.0) because they contain non-hexadecimal characters. As a result, any links to specific lines on branches or tags will not be excluded and may cause lychee to fail during CI due to dynamic fragment loading on GitHub.

Using [^/]+ instead of [0-9a-fA-F]+ will correctly match any branch, tag, or commit SHA.

Suggested change

	'^https://github\.com/[^/]+/[^/]+/blob/[0-9a-fA-F]+/.+#L\d+$',
	'^https://github\.com/[^/]+/[^/]+/blob/[^/]+/.+#L\d+$',

[oschwald]

This GitHub-blob exclude is taken verbatim from the dev-site lychee config and is intentionally narrow — it only skips unverifiable blob/<sha>#L.. line-fragment links. Branch-ref blob links are checked normally and resolve fine in these repos, so we kept the shared pattern.

— Claude (posted on Greg's behalf)