Use floating pragma for IApplication and IStaking interfaces

Pragma statements are fine to float when a contract is intended for
consumption by contracts from another project. This is the case for
IApplication and IStaking that are imported from tBTC v2 contracts.
If we do not use the floating pragma, all tBTC v2 contracts would need
to stay on 0.8.9 and now use the more recent Solidity versions.

Author: pdyraga

contracts/staking/IApplication.sol

@@ -13,7 +13,7 @@
 //               ▐████▌    ▐████▌
 //               ▐████▌    ▐████▌
 
-pragma solidity 0.8.9;
+pragma solidity ^0.8.9;
 
 /// @title  Application interface for Threshold Network applications
 /// @notice Generic interface for an application. Application is an external

contracts/staking/IStaking.sol

@@ -13,7 +13,7 @@
 //               ▐████▌    ▐████▌
 //               ▐████▌    ▐████▌
 
-pragma solidity 0.8.9;
+pragma solidity ^0.8.9;
 
 /// @title Interface of Threshold Network staking contract
 /// @notice The staking contract enables T owners to have their wallets offline