[SC-861] Migrate to new hardhat-ethers

[byshape]

To support ethers v6, hardhat-ethers and some other dependencies need to be updated. In addition to updating the versions of the relevant dependencies, changes have been made to .js files that used the functionality of ethers v5, which has been changed in the new version.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7ab36bb) 83.33% compared to head (67f0cdb) 83.33%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   83.33%   83.33%           
=======================================
  Files           3        3           
  Lines          78       78           
  Branches       13       13           
=======================================
  Hits           65       65           
  Misses         13       13           

Files Changed	Coverage Δ
contracts/CumulativeMerkleDrop.sol	100.00% <100.00%> (ø)
contracts/CumulativeMerkleDrop128.sol	100.00% <100.00%> (ø)
contracts/SignatureMerkleDrop128.sol	69.04% <100.00%> (ø)

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