CI: use Visual Studio 2025 runners instead of 2019

[tautschnig]

Visual Studio 2019 runners are deprecated and will cease to be provided by 2025-06-30.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (34dc8dc) to head (3b75303).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8660   +/-   ##
========================================
  Coverage    80.36%   80.36%           
========================================
  Files         1686     1686           
  Lines       206981   206981           
  Branches        73       73           
========================================
  Hits        166337   166337           
  Misses       40644    40644           

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[tautschnig]

@peterschrammel This one will also require an adjustment to the list of CI tasks that are marked "required".

[kroening]

@peterschrammel This one will also require an adjustment to the list of CI tasks that are marked "required".

Removed the VS 2019 job from the "required" list.