contracts-bedrock: relax semver solidity version

Allows contracts to import semver that are at least solidity
version 0.8.0. This allows a wider range of contracts to be
able to use the `Semver` contract, which doesn't need to
be strictly at a higher version.

Fixes build issues in https://github.com/ethereum-optimism/optimism/pull/4879

packages/contracts-bedrock/contracts/universal/Semver.sol

 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.15;
+pragma solidity ^0.8.0;
 
 import { Strings } from "@openzeppelin/contracts/utils/Strings.sol";