contrib: use hkps://keys.openpgp.org to retrieve builder keys

hkps://hkps.pool.sks-keyservers.net is essentially no-longer functional,
and a number of distributions and GPG tools have since switched to using
this key server as their default.

See this Debian patch for additional context:
https://salsa.debian.org/debian/gnupg2/-/blob/debian/main/debian/patches/Use-hkps-keys.openpgp.org-as-the-default-keyserver.patch

Switch to using keys.openpgp.org in the CI as well.

contrib/verify-commits/README.md

@@ -40,7 +40,7 @@ Import trusted keys
 In order to check the commit signatures, you must add the trusted PGP keys to your machine. [GnuPG](https://gnupg.org/) may be used to import the trusted keys by running the following command:
 
 ```sh
-gpg --keyserver hkp://keyserver.ubuntu.com --recv-keys $(<contrib/verify-commits/trusted-keys)
+gpg --keyserver hkps://keys.openpgp.org --recv-keys $(<contrib/verify-commits/trusted-keys)
 ```
 
 Key expiry/revocation