Deprecate unsafe_html

[srawlins]

I don't think this rule has provided any value outside Google. It is still the only subclass of SecurityLintRule. We can move it wholesale to be an internal rule, and reduce the maintenance burden as a public rule.

• Remove unsafe_html lint rule flutter/flutter#157531
• Remove unsafe_html lint rule marcglasberg/assorted_layout_widgets#34
  • Bump assorted_layout_widgets to e32fabf854927bb4c0c0284624bf30c20c4aab8d flutter/tests#422
• Remove unsafe_html lint rule flutter/cocoon#3982
• Disable three lint rules which are being removed. flutter/cocoon#3991
  • Bump flutter_cocoon to ded610ea7563cff50efa6efaeeb14d69848382c0 flutter/tests#423
• Disable two lint rules which are being removed. Goddchen/dart-rfb#26
  • Disable the dart_rfb and flutter_rfb tests flutter/tests#425
• Disable two lint rules which are being removed. flutter/devtools#8488
• Disable two lint rules which are being removed. Goddchen/flutter-rfb#24
  • Disable the dart_rfb and flutter_rfb tests flutter/tests#425
• Disable two lint rules which are being removed. fzyzcjy/flutter_portal#117
  • Bump flutter_portal to 5584f9bbd5f7dcbc46e3293e395978466c83a453 flutter/tests#424
• Remove flutter_svg tests flutter/tests#419
• Disable two lint rules which are being removed. flutter/packages#7959

[srawlins]

WDYT, @bwilkerson @pq