Bump CBMC version to 5.13.0

[hannes-steffenhagen-diffblue]

In preparation for this weeks release, bump CBMC version. Hopefully PRs like this won’t be necessary anymore once #5456 is merged.

• 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 #5467 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5467   +/-   ##
========================================
  Coverage    68.25%   68.25%           
========================================
  Files         1180     1180           
  Lines        97722    97722           
========================================
  Hits         66698    66698           
  Misses       31024    31024           

Flag	Coverage Δ
#cproversmt2	42.81% <ø> (ø)
#regression	65.42% <ø> (ø)
#unit	32.23% <ø> (ø)

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[chrisr-diffblue]

I'm not sure if he 'semantic meaning of version numbers' discussion has concluded or not, but just wondering why this is 5.13.0 rather than 5.12.7? Might be worth an explanation in the commit message and PR description.

Happy for it to go in either way.