Remove PUT endpoint, update to always use POST for setup experience scripts

[JordanMontgomery]

Related issue: Resolves #35309

Followup changes, see https://fleetdm.slack.com/archives/C019WG4GH0A/p1763137466439419 for more context. We decided not to use the initially proposed PUT endpoint at all and update the existing POST endpoint to have the desired behavior

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.

• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)

• If paths of existing endpoints are modified without backwards compatibility, checked the frontend/CLI for any necessary changes

Testing

• Added/updated automated tests

• Where appropriate, automated tests simulate multiple hosts and test for host isolation (updates to one hosts's records do not affect another)

• QA'd all new/changed functionality manually

[codecov]

Codecov Report

❌ Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.23%. Comparing base (33510dc) to head (a6bb494).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
server/datastore/mysql/setup_experience.go	63.63%	2 Missing and 2 partials ⚠️
ee/server/service/setup_experience.go	50.00%	0 Missing and 1 partial ⚠️
server/service/setup_experience.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35818      +/-   ##
==========================================
- Coverage   66.23%   66.23%   -0.01%     
==========================================
  Files        2109     2109              
  Lines      179221   179203      -18     
  Branches     7443     7443              
==========================================
- Hits       118712   118694      -18     
+ Misses      49628    49626       -2     
- Partials    10881    10883       +2     

Flag	Coverage Δ
backend	67.89% <68.42%> (-0.01%)	⬇️

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