fix: Bump to latest Solidity version (0.8.28)

Earlier versions have known vulnerabilities.

Slither output:
```
INFO:Detectors:
Version constraint ^0.8.13 contains known severe issues (https://solidity.readthedocs.io/en/latest/bugs.html)
	- VerbatimInvalidDeduplication
	- FullInlinerNonExpressionSplitArgumentEvaluationOrder
	- MissingSideEffectsOnSelectorAccess
	- StorageWriteRemovalBeforeConditionalTermination
	- AbiReencodingHeadOverflowWithStaticArrayCleanup
	- DirtyBytesArrayToStorage
	- InlineAssemblyMemorySideEffects
	- DataLocationChangeInInternalOverride
	- NestedCalldataArrayAbiReencodingSizeValidation.
It is used by:
	- ^0.8.13 (src/Counter.sol#2)
Reference: https://github.com/crytic/slither/wiki/Detector-Documentation#incorrect-versions-of-solidity
INFO:Slither:foundry/ analyzed (2 contracts with 93 detectors), 1 result(s) found
```

foundry/test/Counter.t.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: UNLICENSED
-pragma solidity ^0.8.13;
+pragma solidity ^0.8.28;
 
 import {Test, console} from "forge-std/Test.sol";
 import {Counter} from "../src/Counter.sol";