chore: bump interface version to WoW 12.1.0

[Rubyj]

Summary of Changes

Bumps addon compatibility to WoW 12.1.0 (build 69273).

• CooldownTracker.toc: ## Interface: 120007 → 120100, ## Version: 1.7.3 → 1.8.0
• Updated stale Interface 120001 references in README.md, curseforge_description.md, and .github/pull_request_template.md (these were already out of date before this change)

No Lua files were touched — this is a compat/version bump only.

Testing

Non-functional (TOC/docs only). Recommend a quick in-game smoke test on 12.1.0: /cdt to open the tracker, click a row to start a timer, open the settings panel.

Checklist

• Tested in-game on Midnight (Interface 120100)
• No new global variables introduced (or .luacheckrc updated if needed)
• No taint-unsafe patterns introduced (no runtime CreateFrame, SetParent, or SetScript on secure frames)
• Data.lua spell entries include both duration and defaultDuration
• New dev-only files added to .pkgmeta ignore list
• ## Version: in CooldownTracker.toc bumped if this is a release