Delete string_constantt::get_value and string_constantt::set_value functions

[NlightNFotis]

Marked DEPRECATED over the value() function.

Depends on #7999

• 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).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.28%. Comparing base (8bab263) to head (9299bc8).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8003      +/-   ##
===========================================
+ Coverage    78.23%   78.28%   +0.05%     
===========================================
  Files         1722     1722              
  Lines       188692   188556     -136     
  Branches     18452    18459       +7     
===========================================
- Hits        147615   147612       -3     
+ Misses       41077    40944     -133     

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