ref(onboarding): Remove unsupported serverlesscloud (backend)

[s1gr1d]

Remove already hidden onboarding guide for Serverless Cloud. This package does not exist anymore: https://www.serverless.com/blog/serverless-cloud-spins-off-as-ampt

ref getsentry/sentry-docs#7718

Merge frontend PR before: #70719

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.03%. Comparing base (d4f3c6a) to head (f7714fd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #70720      +/-   ##
==========================================
+ Coverage   75.84%   80.03%   +4.18%     
==========================================
  Files        6504     6505       +1     
  Lines      290724   290784      +60     
  Branches    50107    50109       +2     
==========================================
+ Hits       220495   232718   +12223     
+ Misses      69790    57629   -12161     
+ Partials      439      437       -2     

Files	Coverage Δ
...ynamic_sampling/rules/helpers/time_to_adoptions.py	100.00% <ø> (ø)
src/sentry/models/project.py	97.35% <ø> (+7.28%)	⬆️

... and 822 files with indirect coverage changes