test: remove gulp public-api:update docs

[andrewseguin]

Since #22639, the command is no longer provided.

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "continuous-integration/travis-ci/push" is failing
     status "continuous-integration/travis-ci/pr" is failing
     1 pending code review

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[angular-automatic-lock-bot]

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