util: 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 80.79470% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 79.64%. Comparing base (d7b229e) to head (2b2d337).
Report is 2 commits behind head on develop.

Files	Patch %	Lines
src/util/simplify_expr.cpp	68.88%	14 Missing ⚠️
src/util/simplify_expr_struct.cpp	78.12%	7 Missing ⚠️
src/util/std_expr.cpp	44.44%	5 Missing ⚠️
src/util/pointer_offset_size.cpp	96.29%	1 Missing ⚠️
src/util/simplify_expr_pointer.cpp	85.71%	1 Missing ⚠️
src/util/std_types.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8236   +/-   ##
========================================
  Coverage    79.64%   79.64%           
========================================
  Files         1684     1684           
  Lines       195670   195756   +86     
========================================
+ Hits        155841   155912   +71     
- Misses       39829    39844   +15     

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