Update supported-networks.mdx

`Celo-Alfajores` and `Arbitrum Goerli` are supported in the studio, but not on the decentralized network.

Author: Web3Slimchance

pages/en/developing/supported-networks.mdx

@@ -9,10 +9,8 @@ The following networks are supported on The Graph's Decentralized Network:
 - `mainnet` (Ethereum)
 - `gnosis`\*
 - `celo`\*
-- `celo-alfajores`\*
 - `avalanche`\*
 - `arbitrum-one`\*
-- `arbitrum-goerli`\*
 
 \*In beta.