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.

ci/lint/06_script.sh

@@ -25,7 +25,7 @@ test/lint/lint-all.sh
 
 if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ -n "$CIRRUS_CRON" ]; then
     git log --merges --before="2 days ago" -1 --format='%H' > ./contrib/verify-commits/trusted-sha512-root-commit
-    ${CI_RETRY_EXE}  gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys $(<contrib/verify-commits/trusted-keys) &&
+    ${CI_RETRY_EXE}  gpg --keyserver hkps://keys.openpgp.org --recv-keys $(<contrib/verify-commits/trusted-keys) &&
     ./contrib/verify-commits/verify-commits.py --clean-merge=2;
 fi
 

contrib/builder-keys/README.md

@@ -20,7 +20,7 @@ To fetch keys of builders and active developers, feed the list of fingerprints
 of the primary keys into gpg:
 
 ```sh
-while read fingerprint keyholder_name; do gpg --keyserver hkp://subset.pool.sks-keyservers.net --recv-keys ${fingerprint}; done < ./keys.txt
+while read fingerprint keyholder_name; do gpg --keyserver hkps://keys.openpgp.org --recv-keys ${fingerprint}; done < ./keys.txt
 ```
 
 Add your key to the list if you provided Guix attestations for two major or

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