Address various linter (staticcheck, modernize, etc.) comments.

[grant-he]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug

/kind cleanup

/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it: this PR seeks to update the external-attacher repo so various Go linters no longer
raise any warnings for the code. The repo should follow modern Go constructs and best practices, as well as migrate from deprecated symbols and package imports.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer: I am happy to explain any part of my change.

Does this PR introduce a user-facing change?: no

NONE

[k8s-ci-robot]

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[k8s-ci-robot]

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[jsafrane]

/ok-to-test

[jsafrane]

Thanks for your contribution!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grant-he, jsafrane

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