Revert change of StdSignature PubKey to []byte

[aaronc]

In #5795, StdSignature.PubKey was change from crypto.PubKey to []byte. This was supposed to be a temporary workaround for protobuf migration. It breaks legacy clients and should be reverted.

In a separate PR we will also need to revert #5682 and find an alternate solution.

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[codecov]

Codecov Report

Merging #6885 into master will decrease coverage by 2.27%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #6885      +/-   ##
==========================================
- Coverage   60.83%   58.56%   -2.28%     
==========================================
  Files         380      356      -24     
  Lines       24832    21324    -3508     
==========================================
- Hits        15107    12488    -2619     
+ Misses       8568     7852     -716     
+ Partials     1157      984     -173