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 Oct 1, 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 this job we need to update config files relating to the supported
testnets in the `ci` repo.

config/env/sepolia.env

@@ -0,0 +1,3 @@
+GCR_REGISTRY_URL=gcr.io
+GOOGLE_PROJECT_ID=keep-test-f3e0
+NETWORK_ID=11155111