Update to Phoenix v1.8

[josevalim]

• put_secure_browser_headers was updated and we need to update it accordingly (Fix embedding livebook inside iframe with LIVEBOOK_WITHIN_IFRAME #3024)
• We should remove app.html.heex: https://github.com/livebook-dev/livebook/blob/main/lib/livebook_web/components/layouts/app.html.heex