ha.pool.sks-keyservers.net in non-hkps-enabled software

That’s “a high-availibility subset of the pool that require all servers to be identified as a clustered setup”, according to https://sks-keyservers.net/overview-of-pools.php. Shall we use it for e.g. Seahorse to improve robustness (both for users and the test suite, see e.g. #9518 (closed))? (Presumably for hkps-enabled software, we want to keep on using hkps://hkps.pool.sks-keyservers.net). Note that on https://sks-keyservers.net/status/, I see that there are only 4 “servers” (presumably, “clusters”) in that pool.

Related issues

Related to #9346 (closed)

Original created by @intrigeri on 9530 (Redmine)

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