Bump cbmc version to 5.15.0

[NlightNFotis]

This is performing the regular, weekly release we make to CBMC.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5510 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5510   +/-   ##
========================================
  Coverage    68.34%   68.34%           
========================================
  Files         1187     1187           
  Lines        98090    98090           
========================================
  Hits         67044    67044           
  Misses       31046    31046           

Flag	Coverage Δ
#cproversmt2	42.96% <ø> (ø)
#regression	65.49% <ø> (ø)
#unit	32.25% <ø> (ø)

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