Revise cookie 'secure flag' enable condition
The localhost is 'potentially trustworthy' and RFC 6265 allows setting secure flag in this case.
Also check `X-Forwarded-Proto` header value to support reverse proxy usage.
Note: for reverse proxy users, now the `X-Forwarded-Proto` header is expected to be sent to qbt
otherwise the `secure` flag might be set erroneously.
https://datatracker.ietf.org/doc/html/rfc6265#section-4.1.2.5
https://w3c.github.io/webappsec-secure-contexts/#is-origin-trustworthy
Closes #21250.
PR #21260.