Move ShowExtensionSurveyPrompt experiment to ExP

[kimadeline]

For #15530

Note that we explicitly don't migrate people who opted in/out of the old experiment.

[codecov-io]

Codecov Report

Merging #15693 (c3c5dd5) into main (c7fea18) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##            main   #15693   +/-   ##
======================================
- Coverage     64%      64%   -1%     
======================================
  Files        557      557           
  Lines      26814    26812    -2     
  Branches    3901     3901           
======================================
- Hits       17366    17364    -2     
  Misses      8714     8714           
  Partials     734      734           

Impacted Files	Coverage Δ
src/client/activation/extensionSurvey.ts	95% <100%> (-1%)	⬇️
src/client/common/experiments/groups.ts	100% <100%> (ø)
src/client/common/experiments/manager.ts	85% <100%> (ø)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64%. Comparing base (c7fea18) to head (c3c5dd5).
⚠️ Report is 2303 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #15693   +/-   ##
======================================
- Coverage     64%      64%   -1%     
======================================
  Files        557      557           
  Lines      26814    26812    -2     
  Branches    3901     3901           
======================================
- Hits       17366    17364    -2     
  Misses      8714     8714           
  Partials     734      734           

Files with missing lines	Coverage Δ
src/client/activation/extensionSurvey.ts	95% <100%> (-1%)	⬇️
src/client/common/experiments/groups.ts	100% <100%> (ø)
src/client/common/experiments/manager.ts	85% <100%> (ø)

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