Remove the deprecated string type

[Leo6Leo]

          Not sure if this is the best place to tackle it, but while you are fixing the logic in this function it might be worth switching `expectedCeTypes` from type `sets.String` to `Set[string]{}` as `sets.String` is now deprecated

Originally posted by @Cali0707 in #7166 (comment)

[Cali0707]

For this issue, we need to replace sets.String with Set[string]{} in the rekt tests (found in the test/rekt folder).

[Cali0707]

/good-first-issue

[Leo6Leo]

/triage accepted

[knative-prow]

@Cali0707:
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[Iiqbal2000]

Can I take this?

[Leo6Leo]

/assign @Iiqbal2000

[Leo6Leo]

@Iiqbal2000 Please let us know if you have any questions or need any help! Feel free to @Cali0707 and @Leo6Leo

[GetRohitansh]

/assign