Switch back to gcr 3

Balló Györgyrequested to merge
bgyorgy/pinentry:revert-gcr into main

The gcr-4 library does not provide a UI itself, rather rely on the D-Bus interface org.gnome.keyring.SystemPrompter, which is provided by gnome-shell and gcr 3 (gcr-prompter). So users outside of GNOME need to install the gcr 3 package anyway. pinentry-gnome3 can be linked with both gcr 3 and gcr-4, so just build with gcr 3 for now.

Closes: #7 (closed)

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