Release CBMC 6.8.0

[tautschnig]

This release extends the JSON interface to reading full GOTO functions (and not just symbol tables) (via #8713). Also included are various usability improvements around contracts instrumentation (see #8697,
#8694).

• 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.
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.93%. Comparing base (2b184cd) to head (06a493d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8727   +/-   ##
========================================
  Coverage    79.93%   79.93%           
========================================
  Files         1698     1698           
  Lines       187698   187698           
  Branches        73       73           
========================================
+ Hits        150034   150037    +3     
+ Misses       37664    37661    -3     

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