build: unbreak build with gcr-3.92
Newer gcr pre-release removed the widgets and therefore the library is not reliant on Gtk anymore, no matter which version. That's good -- we were only using the GcrCertificate, not the widgets.
Details here: gcr#100 (closed)
In fact, we weren't even needing the UI part of gcr-3, just the gcr-base-3 part. That would've not conflicted with Gtk4.
This allows use to simplify things considerably: both gcr-base-3 and gcr-4 are fine for use for either Gtk3 or Gtk4.