eliminate/simplify gulp setup

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Description

we're using gulp but not for anything that's actually useful. consider this the parent issue for these sub-issues:

• appengine_tasks - can be deleted once we delete the appengine demos, not worth messing with until then
• build_tasks - #10305 - most complicated one, actually using a couple of gulp plugins, non-trivial to migrate
• docs_tasks - #10303 - mostly straightforward
• git_tasks - #10302 - extremely straightforward and why is this even gulp to begin with
• helper_tasks - only used by appengine, package, and build but not needed once those are gone/updated, so delete then
• package_tasks - #10300 - mostly straightforward
• test_tasks - #10306 - somewhat straightforward and the most annoying one to use and causing us real problems with nx
• config.mjs - not a gulpfile, can continue to host shared config that plain node scripts might need

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