Configure Etherscan API key for use with @nomiclabs/hardhat-etherscan

In PR #126 (#126)
we've upgraded the `hardhat-deploy` plugin to latest version (^0.11.15). As part
of this change, the way of storing Etherscan API key has changed - to be
different from how the key is stored for @nomiclabs/hardhat-etherscan plugin
use. When we merged those changes from `main` to the current feature branch, we
no longer had a correct format of the Etherscan API key config to use it with
the `@nomiclabs/hardhat-etherscan` plugin. As we don't want to verify contracts
using `hardhat-deploy` plugin anymore, we need to adjust the config to fit
`@nomiclabs/hardhat-etherscan`'s syntax.

Author: michalinacienciala

hardhat.config.ts

@@ -70,10 +70,8 @@ const config: HardhatUserConfig = {
     username: "thesis",
     project: "thesis/threshold-network",
   },
-  verify: {
-    etherscan: {
-      apiKey: process.env.ETHERSCAN_API_KEY,
-    },
+  etherscan: {
+    apiKey: process.env.ETHERSCAN_API_KEY,
   },
   external: {
     contracts: [