Remove obsolete `block-all-mixed-content` CSP directive (#63595)
The `block-all-mixed-content` CSP directive has been deprecated and it
is not recommended to use it anymore.
Furthermore, the `upgrade-insecure-requests` directive is evaluated
before `block-all-mixed-content`, if the former is set, the latter does
nothing.
Reference:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/block-all-mixed-content
Reference: https://www.w3.org/TR/mixed-content/#strict-checking
Co-authored-by: JJ Kasper <jj@jjsweb.site>