GNOME Settings string changes in network security file chooser

Hi,

This is a notification[0] that we are updating some strings in our
file chooser for selecting certificates/keys in the Security tab of
network settings.

https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/1438

The string changes are:

-   DER, PEM, or PKCS#12 private keys (*.der, *.pem, *.p12, *.key)
+  DER, PEM, PKCS#12, or PGP private keys

and

-  DER or PEM certificates (*.der, *.pem, *.crt, *.cer)
+  DER or PEM certificates

We basically removed the text between parentheses.

The rationale is that we don't want users to focus on literal file
suffixes but instead on file types (known mime-types).

Cheers,
Felipe Borges.

[0] String Change Announcement Period: All string changes must be
announced to gnome-i18n@. https://wiki.gnome.org/FortyThree