Update StagingStatusUpdated.code event value to SHA3_256 hash

[sisyphusSmiling]

Closes: #37

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.81%. Comparing base (5f45a7f) to head (f165996).

Files	Patch %	Lines
contracts/MigrationContractStaging.cdc	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ========================================== - Coverage 83.91% 83.81% -0.11% ========================================== Files 6 6 Lines 342 346 +4 ========================================== + Hits 287 290 +3 - Misses 55 56 +1 ```

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[sisyphusSmiling]

Maybe change the parameter code: String of the event StagingStatusUpdated to codeHash: [UInt8; 32] to make the event less stringly typed and reduce the overhead of hex-encoding

Makes sense. cc-ing @bluesign in case his tooling depends on events at all. I think not, but tagging just in case