Support deployment on Sepolia

The Görli testnet currently used by Threshold/Keep for development purposes is
planned to become deprecated with the end of year 2023. The testnet that is
planned to replace it is called
[Holešky](https://github.com/eth-clients/holesky), however it's not yet
available - it's planned it will become widely accessible on Sep 15, 2023
([source](https://everstake.one/blog/new-ethereum-testnet-holesky-all-you-need-to-know-now)).
Switching our infrastructure to support new testnet is quite time consuming, so
moving directly from Görli to Holešky may be quite risky, especially if there
would be some delays in the date of Holešky genesis (not meeting the planned
timelines is not a rare occurrence in the Ethereum space). As a solution, we
decided to switch first to another testnet that is currently live - Sepolia.
This testnet's EOL is planned for 2026, which gives us plenty of time to move to
Holešky before Sepolia gets deprecated.

As part of the process we need to deploy the legacy Keep contracts on Sepolia.
This PR contains changes needed for such deployment of the `keep-core`
contracts.

solidity-v1/migrations/2_deploy_contracts.js

@@ -49,7 +49,14 @@ const KeepVault = artifacts.require("./geyser/KeepVault.sol")
 
 let initializationPeriod = 43200 // ~12 hours
 const dkgContributionMargin = 1 // 1%
-const testNetworks = ["local", "ropsten", "keep_dev", "alfajores", "goerli"]
+const testNetworks = [
+  "local",
+  "ropsten",
+  "keep_dev",
+  "alfajores",
+  "goerli",
+  "sepolia",
+]
 
 module.exports = async function (deployer, network) {
   // Set the stake initialization period to 1 block for local development and testnet.

solidity-v1/truffle-config.js

@@ -32,29 +32,29 @@ module.exports = {
       network_id: 1101,
     },
 
-    ropsten: {
+    goerli: {
       provider: function () {
         return new HDWalletProvider({
           privateKeys: [process.env.CONTRACT_OWNER_ACCOUNT_PRIVATE_KEY],
           providerOrUrl: process.env.CHAIN_API_URL,
         })
       },
       gas: 6000000,
-      network_id: 3,
+      network_id: 5,
       skipDryRun: true,
       networkCheckTimeout: 120000,
       timeoutBlocks: 200, // # of blocks before a deployment times out  (minimum/default: 50)
     },
 
-    goerli: {
+    sepolia: {
       provider: function () {
         return new HDWalletProvider({
           privateKeys: [process.env.CONTRACT_OWNER_ACCOUNT_PRIVATE_KEY],
           providerOrUrl: process.env.CHAIN_API_URL,
         })
       },
       gas: 6000000,
-      network_id: 5,
+      network_id: 11155111,
       skipDryRun: true,
       networkCheckTimeout: 120000,
       timeoutBlocks: 200, // # of blocks before a deployment times out  (minimum/default: 50)