portalHelper: Fix handling of invalid TLS certificates
WebKitGTK 6.0 moved allow_tls_certificate_for_host() from WebContext to NetworkSession. This was not adjusted here when the portal helper was switched from 4.0 to 6.0.
https://github.com/WebKit/WebKit/commit/2d1b53b211b43047b94b7d2ef8a3907f00e724fb
Closes: #7597 (closed)