protect array_typet::subtype and vector_typet::subtype

[kroening]

This protects the deprecated methods array_typet::subtype() and vector_typet::subtype() and replaces the remaining three users by .element_type().

• 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/
• n/a 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

❗ No coverage uploaded for pull request base (develop@3fe510e). Click here to learn what that means.
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #7559   +/-   ##
==========================================
  Coverage           ?   78.49%           
==========================================
  Files              ?     1670           
  Lines              ?   191678           
  Branches           ?        0           
==========================================
  Hits               ?   150464           
  Misses             ?    41214           
  Partials           ?        0           

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_get.cpp	83.25% <0.00%> (ø)
src/util/std_types.h	95.82% <ø> (ø)
src/solvers/strings/string_refinement.cpp	87.98% <50.00%> (ø)

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