Revert "Update fog-core requirement from <= 2.1.0 to <= 2.3.0"

[Temikus]

Reverts #481 due to causing a regression around #422

We need to push out a minor release before we can update that.

[codecov]

Codecov Report

Merging #487 into master will increase coverage by 7.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
+ Coverage   81.41%   88.96%   +7.55%     
==========================================
  Files         339      339              
  Lines        5639     5639              
==========================================
+ Hits         4591     5017     +426     
+ Misses       1048      622     -426     

Impacted Files	Coverage Δ
lib/fog/storage/google.rb	80.00% <0.00%> (ø)	⬆️
lib/fog/google/sql/mock.rb	61.11% <0.00%> (ø)	⬆️
lib/fog/google/sql/real.rb	100.00% <0.00%> (ø)	⬆️
lib/fog/google/pubsub/mock.rb	62.50% <0.00%> (ø)	⬆️
lib/fog/google/pubsub/real.rb	100.00% <0.00%> (ø)	⬆️
lib/fog/google/models/sql/flag.rb	100.00% <0.00%> (ø)	⬆️
lib/fog/google/models/sql/tier.rb	100.00% <0.00%> (ø)	⬆️
lib/fog/google/models/sql/user.rb	96.42% <0.00%> (ø)	⬆️
lib/fog/google/models/sql/flags.rb	100.00% <0.00%> (ø)	⬆️
lib/fog/google/models/sql/tiers.rb	100.00% <0.00%> (ø)	⬆️
... and 162 more

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