chore: Change ownership of SelectedNetworkController (#5312)

## Explanation

The package `@metamask/selected-network-controller` is now owned solely
by the Wallet API Platform team, rather than being shared between
multiple teams. This team has the most context for what this code does,
and it will in the future be increasingly less relevant to the rest of
the wallet apart from supporting the EIP-1193 provider.

## References

N/A

## Changelog

N/A

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

.github/CODEOWNERS

@@ -43,6 +43,7 @@
 ## Wallet API Platform Team
 /packages/multichain                       @MetaMask/wallet-api-platform-engineers
 /packages/queued-request-controller        @MetaMask/wallet-api-platform-engineers
+/packages/selected-network-controller      @MetaMask/wallet-api-platform-engineers
 
 ## Wallet Framework Team
 /packages/base-controller              @MetaMask/wallet-framework-engineers
@@ -62,7 +63,6 @@
 /packages/network-controller           @MetaMask/wallet-framework-engineers @MetaMask/metamask-assets
 /packages/permission-controller        @MetaMask/wallet-api-platform-engineers @MetaMask/wallet-framework-engineers @MetaMask/snaps-devs
 /packages/permission-log-controller    @MetaMask/wallet-api-platform-engineers @MetaMask/wallet-framework-engineers
-/packages/selected-network-controller  @MetaMask/wallet-api-platform-engineers @MetaMask/wallet-framework-engineers @MetaMask/metamask-assets
 /packages/profile-sync-controller      @MetaMask/notifications @MetaMask/identity
 
 ## Package Release related