Add /fd and /td SHA256 as now necessary

[NlightNFotis]

An update to the Windows SDK from build 20236 onwards requires
that the /fd and /td options be specified. This commits sets them
both to SHA256 (recommended) in the Windows release build.

Microsoft documentation on this:
https://docs.microsoft.com/en-us/dotnet/framework/tools/signtool-exe

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6349 (aca7eba) into develop (e4ee1ed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6349   +/-   ##
========================================
  Coverage    75.90%   75.90%           
========================================
  Files         1515     1515           
  Lines       163990   163990           
========================================
  Hits        124469   124469           
  Misses       39521    39521           

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