chore: Move Ankr to default ETH Sepolia services (#358)

([XC-272](https://dfinity.atlassian.net/browse/XC-272)) Move Ankr to
default ETH Sepolia services

Since Ankr seems to have fixed their IPv6 compatibility, move it to the
list of default ETH Sepolia services, and move rpc.sepolia.org to
non-default ETH Sepolia providers.

[XC-272]:
https://dfinity.atlassian.net/browse/XC-272?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

src/rpc_client/mod.rs

@@ -72,12 +72,12 @@ impl Providers {
     ];
 
     const DEFAULT_ETH_SEPOLIA_SERVICES: &'static [EthSepoliaService] = &[
-        EthSepoliaService::Sepolia,
+        EthSepoliaService::Ankr,
         EthSepoliaService::BlockPi,
         EthSepoliaService::PublicNode,
     ];
     const NON_DEFAULT_ETH_SEPOLIA_SERVICES: &'static [EthSepoliaService] =
-        &[EthSepoliaService::Alchemy, EthSepoliaService::Ankr];
+        &[EthSepoliaService::Alchemy, EthSepoliaService::Sepolia];
 
     const DEFAULT_L2_MAINNET_SERVICES: &'static [L2MainnetService] = &[
         L2MainnetService::Llama,