Couple `snap_manageAccounts` to `endowment:keyring`

[FrederikBolding]

snap_manageAccounts is only supposed to be used by Snaps that implement the keyring API, thus we don't need two separate permissions for this.

We should figure out a way to make snap_manageAccounts not a permission anymore, but instead an RPC method gated by endowment:keyring. E.g. https://github.com/MetaMask/snaps/blob/a4f3df3a7eef8c8dc0eea92b0f952471e14f0c1f/packages/snaps-rpc-methods/src/permitted/experimentalProviderRequest.ts#L155-L157

This needs to be done in a backwards compatible way.