Enabled modernize linter configuration.

[atombrella]

Spawned from #3399 which is too messy to clean up because I made some mistakes with git.
@Stevenjin8 @fjglira Can you please add the ok-to-test label, so this one could land more quickly. Thank you!

Modernize is a golang tool that contains many built-in replacements to use newer golang semantics and standard library features. The PR enables the rules that have been dealt with in the istio repository.

The entire list for GoLang 1.26 is in the documentation.
https://golangci-lint.run/docs/linters/configuration/#modernize

• forvar. I believe it's already covered by copyloopvar, so it's easier to just enable it here.
• rangeint (currently on its way to being enforced in istio in a PR).
• Some strings-rules
• slicescontains and slicessort have only been dealt with in batches. These rules are also quite neat to simplify code. I fixed the few cases of slicescontains that modernize found. It also removed one internal function.

[istio-testing]

Hi @atombrella. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[fjglira]

/ok-to-test