The keyring/ directory contains some gpg keyring files that we use
during the build to verify gpg signatures on downloaded files, or git
tags. In order to be able to continue to use a git tag even after the
key or sub-key that signed it expired (which is common when one is
rotating sub-keys frequently), we configured gpg to ignore key expirations
when verifying git tag signatures. However this also means that we should
make sure that our keyring files do not contain expired keys or subkeys
that are not supposed to be used anymore.

This directory contains some scripts that can help clean the keyring
files.

The complete process for cleaning keyring files starts with:

 - Run `list-all-keyrings` to see if we include any expired key or sub-key.

Then for each expired key or sub-key:

 - Check if the expiration is expected, and do nothing in that case.

 - Check if the owner of that key or sub-key extended it, and in that
   case add the updated key or sub-key.

 - If a key is not needed anymore (but other keys in the keyring are
   still needed), remove it with `gpg --delete-keys <key>`.

 - If a sub-key is not needed anymore, but the main key still contains
   at least one other valid sub-key, use `drop-expired-sub-keys` to
   remove the expired sub-key.
