Update heroku/heroku Docker tag to v26

[openssf-badge-renovate]

This PR contains the following updates:

Package	Type	Update	Change
heroku/heroku	docker	major	24-build → 26-build

This changes the operating system everything runs on.

Merging it moves the test environment to the new stack. When

it reaches staging, the deploy job sets that application to

the same stack and rebuilds; production follows when it is

advanced. Maintenance mode is taken automatically for a stack

change.

Rolling back means reverting this pull request and letting a

full deploy move the stack back.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (73db726) to head (bbcfe72).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2948   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           82        82           
  Lines         4576      4576           
=========================================
  Hits          4576      4576           

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