Deprecate support for the rpmdb keystore

[pmatilai]

Once we have an alternative keystore that supports all the operations the current rpmdb version does, switch the default over to that and deprecate support for the rpmdb gpg-pubkeys.

[mlschroe]

Hey, I like the rpmdb keystore!

[pmatilai]

We don't 😅

But after further consideration, this is probably too big and abrupt a change to do just like that anyhow, and if we force it at 6.0 it would probably just hurt its adoption. So, we'll go the long route: switch to another equally or more capable keystore by default, but support the rpmdb version for ... some to-be-determined time. Probably, for quite a while as these things tend to go.