Switch gpg.mozilla.org out for keys.openpgp.org

[ajvb]

• Make use of keys.openpgp.org
• No longer allow for configuring a different keyserver.

Short-term fix for #727 and currently broken builds.

[codecov-commenter]

Codecov Report

Merging #732 into develop will increase coverage by 0.68%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           develop     #732      +/-   ##
===========================================
+ Coverage    36.44%   37.12%   +0.68%     
===========================================
  Files           22       22              
  Lines         3205     3418     +213     
===========================================
+ Hits          1168     1269     +101     
- Misses        1918     2023     +105     
- Partials       119      126       +7     

Impacted Files	Coverage Δ
pgp/keysource.go	34.08% <75.00%> (+1.18%)	⬆️
sops.go	55.19% <0.00%> (-1.18%)	⬇️
stores/stores.go	0.00% <0.00%> (ø)
config/config.go	74.62% <0.00%> (+3.39%)	⬆️

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