fix: use sepolia bootnodes in sepolia config

[dieutx]

Summary

• replace the Base Sepolia consensus bootnodes in .env.sepolia with the chain-correct ENRs
• update the optional Sepolia snap-sync execution bootnode examples to the matching Sepolia enodes
• align the sample config with the authoritative Sepolia bootnode list in base/base

Problem

base/node was shipping .env.sepolia with the same bootnode values as .env.mainnet.

That means the default Sepolia config advertised mainnet consensus ENRs whose opstack payload resolves to chain ID 8453, while Sepolia expects 84532. Operators in issue #1041 reported these ENRs being rejected as InvalidChainId(8453), leaving discovery with zero usable bootstrap peers.

Solution

Use the Sepolia bootnode set from base/base's chain config:

• consensus ENRs from crates/common/chains/src/config.rs for chain 84532
• matching Sepolia execution bootnodes for the optional commented snap-sync examples

Verification

• confirmed .env.sepolia previously matched .env.mainnet bootnodes byte-for-byte
• confirmed base/base has a distinct Sepolia bootnode set for chain 84532
• updated .env.sepolia to match those authoritative Sepolia values

Closes #1041

[cb-heimdall]

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