goto-symex: 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 75.86207% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 79.63%. Comparing base (d7b229e) to head (b6d7790).
Report is 2 commits behind head on develop.

Files	Patch %	Lines
src/goto-symex/auto_objects.cpp	0.00%	8 Missing ⚠️
src/goto-symex/shadow_memory_util.cpp	80.00%	4 Missing ⚠️
src/goto-symex/field_sensitivity.cpp	88.88%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8222      +/-   ##
===========================================
- Coverage    79.64%   79.63%   -0.01%     
===========================================
  Files         1684     1684              
  Lines       195670   195703      +33     
===========================================
+ Hits        155841   155857      +16     
- Misses       39829    39846      +17     

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