Update lefthook installation guide

What does this MR do and why?

"Style guide" page contains the section "Pre-push static analysis with Lefthook":
https://docs.gitlab.com/ee/development/contributing/style_guides.html#pre-push-static-analysis-with-lefthook

The described section includes the installation guide with the following command:
bundle exec lefthook run prepare-commit-msg
But this command does not do anything.

Most likely because prepare-commit-msg hook is not setup in lefthook.yml.
This MR changes lefthook's test command to use pre-push git hook instead.

Screenshots or screen recordings

The output of bundle exec lefthook run pre-push:

MR acceptance checklist

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