ci(renovate): auto-bump go-github in examples and readme

[cbrgm]

What

Make Renovate bump the go-github version everywhere, not just the top-level go.mod.

• postUpdateOptions: ["gomodUpdateImportPaths"] -> on a major bump, Renovate rewrites the Go import paths (.../v89/github -> .../v90/github) in every module: the examples/* modules, the hand-written githubevents/dispatch_test.go, and the generated files.
• a customManagers regex that keeps the README's import snippet in sync, tracked against go-github's github tags.

Why

Right now a go-github major bump only edits go.mod. The import paths in examples/*/*.go and the README code block still point at the old major, so they get bumped by hand every time. gomodUpdateImportPaths is the built-in Renovate feature for exactly this (it runs the mod tool per module), so all the .go files move automatically. The README isn't a Go file, so it needs the small regex manager.

Note on the examples: they depend on a published githubevents/v2 (no local replace), so they can only move to a new go-github major once a githubevents release using that major exists. After that release, Renovate bumps the githubevents dep and go-github together, and this config rewrites the import paths for you.

Testing

$ renovate-config-validator .github/renovate.json
 INFO: Config validated successfully

The README manager validates against the schema. Since go's semantic-import-versioning (major-in-path) is a bit awkward for a non-code file, i'd confirm it on the next Renovate dry-run / first real bump, it's doc-only so nothing breaks if it needs a tweak.

Checklist

• Tests added/updated (config validated; behavior is Renovate-side)
• No breaking changes (CI/tooling config only)
• Readable commit history (1 commit)
• AI code review considered and comments resolved