Co-authored-by: Jongsun Suh <jongsun.suh@icloud.com>

## Explanation

This PR replaces the reference to OP goerli to OP sepolia. It also
renames the variable `OPTIMISM_TESTNET` to `OPTIMISM_SEPOLIA`.

Renaming the variable and switching to Sepolia is a breaking change,
please use Sepolia and `OPTIMISM_SEPOLIA` instead of `OPTIMISM_TESTNET`.

## References

* Fixes MetaMask/mobile-planning#1569
* Related to
[#67890](MetaMask/metamask-mobile#8784)

## Changelog

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### `@metamask/transaction-controller`

**Removed**

- **BREAKING**: Renamed `OPTIMISM_TESTNET` to `OPTIMISM_SEPOLIA` and
updated the chainId accordingly.
- **BREAKING**: Updated the etherscan subdomain that was for
OPTIMISM_TESTNET from goerli to sepolia.

### `@metamask/preferences-controller`

**Removed**

- **BREAKING**: Renamed `OPTIMISM_TESTNET` to `OPTIMISM_SEPOLIA` and
updated the chainId accordingly.

### `@metamask/name-controller`

**Removed**

- **BREAKING**: Renamed `OPTIMISM_TESTNET` to `OPTIMISM_SEPOLIA` and
updated the chainId accordingly.
- **BREAKING**: updated the etherscan subdomain for OPTIMISM_TESTNET
from goerli to sepolia

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

…ther changelogs