[ci] Switch from impi to gci linter

[mx-psi]

Component(s)

No response

Describe the issue you're reporting

We currently use impi for enforcing Go import order. An alternative called gci exists, which has the following advantages:

• builtin on golangci-lint (which implies no need for a separate binary/test step and possibly faster results),
• higher repo activity (commited in the last ~month, recent release, higher number of stars and forks)
• supports auto-fixing

We should replace our usage of impi by gci on this repository.

[github-actions]

Pinging code owners:

• ci: @jpkrohling

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[atoulme]

I need this! I can take it.