stricter hostname validation and replace

[kradalby]

fixes #2762

  Client Registration (New Nodes)

  'my-laptop'       → 'my-laptop'        ✓ Valid, accepted as-is
  'My-Laptop'       → 'my-laptop'        ✓ Uppercase → lowercase
  'my_laptop'       → 'invalid-a1b2c3'   ✗ Contains underscore
  'test@host'       → 'invalid-d4e5f6'   ✗ Contains @
  'server-北京-01'  → 'invalid-g7h8i9'   ✗ Contains Chinese chars
  'laptop-🚀'       → 'invalid-j1k2l3'   ✗ Contains emoji
  'very-long-hostname-that-exceeds-sixty-three-characters-and-more'   → 'invalid-m4n5o6'   ✗ Too long (>63 chars)

  Hostinfo Updates (From Tailscale Client) or CLI

  'my-laptop'       → Accepted           ✓ Valid
  'My-Laptop'       → 'my-laptop'        ✓ Lowercased and accepted
  'my_laptop'       → Rejected            ✗ "invalid characters"
  'test@host'       → Rejected            ✗ "invalid characters"
  'server-北京-01'  → Rejected            ✗ "invalid characters"
  '-test'           → Rejected            ✗ "cannot start or end with a hyphen"
  'a'               → Rejected            ✗ "must be at least 2 characters"

[nblock]

A entry in the "breaking" section of the changelog would be good as some hostnames that worked before will now be rejected/renamed.