[Docs]: Refresh README to reflect current addon state

[Xerrion]

Problem

The README contains several factually incorrect claims:

• Cataclysm Classic support is documented but does not exist - the TOC has no Interface-Cata line and there is no Core_Cata.lua file
• Lua filenames in the Supported Versions table are wrong - actual files live under Listeners/ as Core_Retail.lua, Core_Mists.lua, Core_BCC.lua, Core_Vanilla.lua
• Configuration says settings persist per-character; the TOC declares regular SavedVariables, so settings are account-wide
• Missing the M+ stop-delay (5-second grace period after CHALLENGE_MODE_COMPLETED, fixed in 1.4.3) that prevents abandoned logs
• Missing the no-dependencies design choice as a selling point (no Ace3, no LibStub)
• ACL auto-enable behavior is mentioned but not detailed

Proposed Solution

Refresh README.md to reflect current state on master:

• Remove all Cataclysm Classic references
• Fix Supported Versions table to use real filenames under Listeners/
• Correct the Configuration scope claim (account-wide, not per-character)
• Add M+ stop-delay behavior to How It Works
• Add a short note about zero external library dependencies
• Add a brief Events Handled section listing the 5 events the addon listens for

No source code changes.

Alternatives

Patch only the Cataclysm claim and filename errors. Rejected: the per-character claim is also actively wrong and would confuse users about where their settings live.