7.0.0: Assets CSP, PHP 7.4+

• Made it possible to use web.frontend.Security instances for assets
  (@thekid)
• Implemented default CSP in web.frontend.AssetsFrom to prevent XSS
  with SVG files. See #49 and https://stackoverflow.com/q/10557137
  (@thekid)
• Heads up: Dropped support for PHP < 7.4, see xp-framework/rfc#343
  (@thekid)
• Added PHP 8.5 to test matrix - @thekid