Lock pragma version for L*BitcoinDepositor contracts (#806)

Depends on: #805

Here we fix the Solidity version to be `0.8.17` which is used across all
tBTC contracts.

solidity/contracts/l2/L1BitcoinDepositor.sol

@@ -13,7 +13,7 @@
 //               ▐████▌    ▐████▌
 //               ▐████▌    ▐████▌
 
-pragma solidity ^0.8.17;
+pragma solidity 0.8.17;
 
 import "@keep-network/random-beacon/contracts/Reimbursable.sol";
 import "@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol";

solidity/contracts/l2/L2BitcoinDepositor.sol

@@ -13,7 +13,7 @@
 //               ▐████▌    ▐████▌
 //               ▐████▌    ▐████▌
 
-pragma solidity ^0.8.17;
+pragma solidity 0.8.17;
 
 import "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";