ci: support no-checkout release approval notices

[cat-ph]

Summary

• keep the release approval Slack workflow's existing checkout behavior by default
• add an opt-in no-checkout compare URL path for callers with version-sortable tag patterns
• require tag_sort=version when checkout_repository=false, because remote creatordate sorting is not compatible with standalone SDK repos

Why

PostHog/posthog release-cli approval notification spent 11m29s in checkout because actions/checkout fetched every branch and tag from the monorepo. The Slack notification only needs a compare URL, but other SDK release workflows still rely on the existing checkout-based latest-tag behavior.

Compatibility

Audited local SDK callers: posthog-ruby, posthog-go, posthog-php, posthog-python, posthog-ios, posthog-rs, posthog-android, posthog-dotnet, posthog-elixir, and posthog-js.

Default behavior remains checkout_repository=true, so existing @main and pinned callers keep the old full-checkout tag lookup semantics.

Test Plan

• actionlint .github/workflows/notify-approval-needed.yml
• python3 YAML parse for .github/workflows/notify-approval-needed.yml
• verified checkout_repository=false with tag_pattern=posthog-cli/v* and tag_sort=version returns posthog-cli/v0.7.20 and master without checkout
• verified checkout_repository=false with tag_sort=creatordate fails loudly
• verified GitHub compare accepts posthog-cli/v0.7.20...master