chore: cleaned up Solidity warnings

Description:

cleaned up all warnings thrown by Solidity after compilation.
fixed unit tests where needed
there're only 4 warnings left but only because they serve the purpose of the coverages

Warning: This declaration shadows an existing declaration.
  --> contracts/solidity/scoping/Scoping.sol:21:13:
   |
21 |             uint x;
   |             ^^^^^^
Note: The shadowed declaration is here:
  --> contracts/solidity/scoping/Scoping.sol:18:9:
   |
18 |         uint x = 1;
   |         ^^^^^^

Warning: "selfdestruct" has been deprecated. The underlying opcode will eventually undergo breaking changes, and its use is not recommended.
  --> contracts/discrepancies/nonce/InternalCallee.sol:27:9:
   |
27 |         selfdestruct(_addr);
   |         ^^^^^^^^^^^^

Warning: "selfdestruct" has been deprecated. The underlying opcode will eventually undergo breaking changes, and its use is not recommended.
  --> contracts/discrepancies/nonce/InternalCaller.sol:56:9:
   |
56 |         selfdestruct(_addr);
   |         ^^^^^^^^^^^^

Warning: "selfdestruct" has been deprecated. The underlying opcode will eventually undergo breaking changes, and its use is not recommended.
  --> contracts/solidity/signature-example/ReceiverPays.sol:27:9:
   |
27 |         selfdestruct(payable(msg.sender));
   |         ^^^^^^^^^^^^

Related issue(s):

Fixes #435

Notes for reviewer:

Checklist

[ ] Documented (Code comments, README, etc.)
[ ] Tested (unit, integration, etc.)

hashgraph / hedera-smart-contracts

chore: cleaned up Solidity warnings #701

Test Results