Documentation: consistently use .gb suffix for goto binaries

[tautschnig]

Although our implementation does not care about the suffix, we should
avoid surprises. We mostly already used .gb as a suffix in
documentation, and consistently do so in regression tests. This commit
fixes the remaining occurrences of .goto in documentation, replacing all
of them by .gb.

• 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

Merging #5826 (05e3ab4) into develop (23c11e8) will increase coverage by 3.76%.
The diff coverage is 95.04%.

@@             Coverage Diff             @@
##           develop    #5826      +/-   ##
===========================================
+ Coverage    69.08%   72.84%   +3.76%     
===========================================
  Files         1242     1423     +181     
  Lines       100842   154063   +53221     
===========================================
+ Hits         69667   112234   +42567     
- Misses       31175    41829   +10654     

Flag	Coverage Δ
regression	?
unit	?

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Impacted Files	Coverage Δ
...ses/variable-sensitivity/interval_abstract_value.h	100.00% <ø> (ø)
...s/variable-sensitivity/value_set_abstract_object.h	100.00% <ø> (ø)
...-sensitivity/variable_sensitivity_object_factory.h	100.00% <ø> (ø)
...riable-sensitivity/full_struct_abstract_object.cpp	81.81% <33.33%> (-0.09%)	⬇️
.../analyses/variable-sensitivity/abstract_object.cpp	69.15% <50.00%> (-4.80%)	⬇️
...yses/variable-sensitivity/abstract_environment.cpp	87.42% <87.50%> (+2.02%)	⬆️
...s/variable-sensitivity/interval_abstract_value.cpp	78.77% <94.44%> (-0.88%)	⬇️
...variable-sensitivity/value_set_abstract_object.cpp	87.01% <97.14%> (+15.58%)	⬆️
...alyses/variable-sensitivity/abstract_environment.h	100.00% <100.00%> (ø)
...s/variable-sensitivity/constant_abstract_value.cpp	94.31% <100.00%> (+2.31%)	⬆️
... and 1380 more

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