Add parent slot to getConfirmedBlock

[CriesofCarrots]

Summary of Changes

• Adds parent slot to RpcConfirmedBlock struct, and populates in blocktree
• Makes getConfirmedBlock rpc fields consistent with camelCase
• Cleans up bad text-replace in method name

Toward #6867

[codecov]

Codecov Report

Merging #7038 into master will decrease coverage by 11.9%.
The diff coverage is 33.3%.

@@           Coverage Diff            @@
##           master   #7038     +/-   ##
========================================
- Coverage    70.7%   58.7%    -12%     
========================================
  Files         225     225             
  Lines       48282   58125   +9843     
========================================
+ Hits        34136   34139      +3     
- Misses      14146   23986   +9840