Switch Fleet-maintained apps to use manifest-based structure

[iansltx]

For #26082.

Checklist for submitter

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

• 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
• If database migrations are included, checked table schema to confirm autoupdate
• For database migrations:
  • Checked schema for all modified table for columns that will auto-update timestamps during migration.
  • Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
  • Ensured the correct collation is explicitly set for character columns (COLLATE utf8mb4_unicode_ci).
• Added/updated automated tests
• 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)
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 63.12292% with 111 lines in your changes missing coverage. Please review.

Project coverage is 63.78%. Comparing base (af40739) to head (0af0381).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
server/mdm/maintainedapps/sync.go	51.11%	34 Missing and 10 partials ⚠️
...re/mysql/migrations/tables/20250320200000_FMAv2.go	62.63%	24 Missing and 10 partials ⚠️
ee/server/service/maintained_apps.go	79.06%	7 Missing and 2 partials ⚠️
server/mdm/maintainedapps/installers.go	0.00%	9 Missing ⚠️
pkg/automatic_policy/automatic_policy.go	57.14%	4 Missing and 2 partials ⚠️
server/fleet/maintained_apps.go	40.00%	4 Missing and 2 partials ⚠️
server/datastore/mysql/maintained_apps.go	81.81%	2 Missing ⚠️
cmd/fleet/cron.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27201      +/-   ##
==========================================
- Coverage   63.83%   63.78%   -0.05%     
==========================================
  Files        1737     1737              
  Lines      165290   165138     -152     
  Branches     4429     4429              
==========================================
- Hits       105507   105340     -167     
- Misses      51563    51570       +7     
- Partials     8220     8228       +8     

Flag	Coverage Δ
backend	64.66% <63.12%> (-0.05%)	⬇️

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[jahzielv]

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