feat: replace Promise.join with promise.all()

[Emmanuel-Melon]

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Please include a description of your change & check your PR against this list, thanks!

• [ ] There's a clear use-case for this code change, explained below
• [ ] Commit message has a short title & references relevant issues
• [ ] The build will pass (run yarn test:all and yarn lint)

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[codecov[bot]]

Codecov Report

Merging #14972 (47f5758) into main (3231863) will increase coverage by 8.50%. The diff coverage is 60.00%.

:exclamation: Current head 47f5758 differs from pull request most recent head b76ffc8. Consider uploading reports for the commit b76ffc8 to get more accurate results

@@            Coverage Diff             @@
##             main   #14972      +/-   ##
==========================================
+ Coverage   52.60%   61.10%   +8.50%     
==========================================
  Files        1392      596     -796     
  Lines       89130    47668   -41462     
  Branches    10131     4293    -5838     
==========================================
- Hits        46884    29127   -17757     
+ Misses      42194    18493   -23701     
+ Partials       52       48       -4     

Impacted Files	Coverage Δ
core/server/models/user.js	49.03% <60.00%> (ø)
...ore/core/server/services/members/exporter/query.js
ghost/core/core/server/lib/mobiledoc.js
ghost/core/core/frontend/helpers/excerpt.js
...core/core/frontend/services/rendering/templates.js
...t/core/core/server/services/posts/posts-service.js
ghost/core/core/server/data/db/state-manager.js
...r/api/endpoints/utils/serializers/input/members.js
ghost/admin/app/components/modal-unsuspend-user.js
ghost/core/core/frontend/web/middleware/index.js
... and 1979 more

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[markstos]

Why is this change considered an improvement? The docs for Promise.join say "While .all is good for handling a dynamically sized list of uniform promises, Promise.join is much easier (and more performant) to use when you have a fixed amount of discrete promises that you want to coordinate concurrently."

Did you check to see which method performed better?

If the the goal was to use a native Promise method instead of a Bluebird method, Bluebird is still required after this change is made, so there's not much benefit there.

[vikaspotluri123]

@markstos see #14882 🙂