Cancel upcoming activities: implement cancelation itself

[mna]

For #27409 (partial)

Checklist for submitter

• [x] Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)
• [x] Added/updated automated tests
• [x] A detailed QA plan exists on the associated ticket (if it isn't there, work with the product group's QA engineer to add it)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 35 lines in your changes missing coverage. Please review.

Project coverage is 63.90%. Comparing base (df61e6b) to head (8edab3a). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
server/datastore/mysql/activities.go	80.11%	25 Missing and 10 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27773      +/-   ##
==========================================
+ Coverage   63.64%   63.90%   +0.26%     
==========================================
  Files        1774     1759      -15     
  Lines      169219   168504     -715     
  Branches     4947     4563     -384     
==========================================
- Hits       107692   107687       -5     
+ Misses      53107    52394     -713     
- Partials     8420     8423       +3     

Flag	Coverage Δ
backend	64.73% <87.50%> (+0.03%)	:arrow_up:

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