Add Certora syntax highlighting

[james-toussaint]

Appearance of most Certora files is not ideal:

• https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/certora/specs/AccessControlDefaultAdminRules.spec

Following update should improve readability:

• https://docs.certora.com/en/latest/docs/user-guide/github_highlighting.html

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