Bump ERC1967Utils pragma to 0.8.21 and set Slither CI version to 0.10.1

[dimitriospapathanasiou]

Fixes #4995

I noticed the related issue about the pragma that is used. After minor local tests, I verified that the increase to 0.8.21 works well.

Same as #4999 but with the proper files added. Excuse my previous PR, I am a first time contributor. Sorry for any inconvenience.

PR Checklist

• Tests
• Documentation
• Changeset entry (run npx changeset add)

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Latest commit: 9d1860b

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[Amxx]

Thank you @dimitriospapathanasiou

We don't need a changeset for that.

We need @ernestognw 's 2nd approval and we are good.

[ernestognw]

Looks good to me as well, I just added a note to the changelog referring to the breaking changes arising from this.

[ernestognw]

CI is broken due to Slither 0.10.2. The rule breaking is pragma but I'm struggling to see why it's failing now.

Let's not unblock this and set the Slither version to 0.10.1 for now.