docs: update deprecated Goerli reference to Sepolia

[Surya512002]

Description

I noticed the "Account Abstraction using Biconomy" tutorial still contained multiple references to the deprecated "Base Goerli" network.

Fix

• Updated all instances of "Base Goerli" to "Base Sepolia".
• Updated RPC URLs and network selection instructions throughout the tutorial.

Context

Goerli is deprecated, so these instructions were outdated for new users.

[cb-heimdall]

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