Remove slither hardcoded version

[ernestognw]

Fixes #4397

After #4346 we fixed the version of slither to 0.9.3 because the unchecked-lowlevel rule was triggering false positives (see crytic/slither#2008). Slither 0.9.6 was just released and includes a fix to the false positives, so we're enabling the latest version again.

[changeset-bot]

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Latest commit: 2444586

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