Seahorse can't sync keys with keyservers: Request Entity Too Large

Spotted by our test suite while working on #12689 (closed). That’s the case:

with keys.openpgp.org’s Onion service (that my branch for #12689 (closed) sets as the default)
in our test suite, when using keys.mayfirst.org as the backend

I can’t trivially reproduce this with hkp://jirk5u4osbsr34t5.onion but I think it depends on the keyserver that the pool will give us: as the keys.mayfirst.org issue shows, this problem is not specific to keys.openpgp.org.

Related issues

Related to #12689 (closed)
Related to #14770 (closed)

Original created by @intrigeri on 17169 (Redmine)

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