Deprecate CSP block-all-mixed-content directive (#7664)
Deprecate CSP block-all-mixed-content directive (#7664)

This change marks the CSP block-all-mixed-content directive as
deprecated:true, per the current version of the CSP spec at
https://w3c.github.io/webappsec-mixed-content/#obsolescences:

> This specification renders the Mixed Content §4 Strict Mixed Content
> Checking mode and the block-all-mixed-content CSP directive obsolete,
> because all mixed content is now blocked if it can’t be autoupgraded.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data