Solvers: Replace uses of namespacet::follow

[tautschnig]

This is deprecated. Use suitable variants of follow_tag instead.

• 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.
• n/a 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

Attention: Patch coverage is 50.54945% with 45 lines in your changes are missing coverage. Please review.

Project coverage is 77.62%. Comparing base (ae7d311) to head (8a0782b).

Files	Patch %	Lines
src/solvers/flattening/bv_pointers.cpp	0.00%	20 Missing ⚠️
src/solvers/smt2/smt2_conv.cpp	50.00%	14 Missing ⚠️
src/solvers/flattening/boolbv_update.cpp	0.00%	8 Missing ⚠️
src/solvers/flattening/boolbv_typecast.cpp	90.00%	1 Missing ⚠️
...vers/smt2_incremental/encoding/struct_encoding.cpp	90.00%	1 Missing ⚠️
src/solvers/strings/string_refinement.cpp	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8235      +/-   ##
===========================================
- Coverage    77.81%   77.62%   -0.20%     
===========================================
  Files         1721     1721              
  Lines       189460   189965     +505     
  Branches     18401    18411      +10     
===========================================
+ Hits        147432   147459      +27     
- Misses       42028    42506     +478     

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