chore: reframe Discord as community, canonicalize support links and X handle

Discord support officially ends 2026-06-15; Discord is positioned as
a community space only.

- Banner/badge URLs: discord.com/invite/b5rXHjAg7A -> clerk.com/discord;
  twitter.com/(clerk|ClerkDev|clerkdev) -> x.com/clerk; shields.io
  Twitter follow badges + intent URLs normalized to x.com handle clerk.
- README "support thread on our [Discord]" boilerplate replaced with
  contact/support + support@clerk.com routing.
- "Find support through our Twitter" line replaced with X follow-only.
- support@clerk.dev -> support@clerk.com; clerk.com/support ->
  clerk.com/contact/support where present.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>