Release CBMC 6.7.0

[tautschnig]

This release adds aarch64 va_list support (via #8572), which makes all tests pass on aarch64 Linux. We reworked expression simplification during symbolic execution (via #8642, #8647, #8627) to produce smaller and quicker-to-solve formulae for scenarios seen by our users.

• 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 80.39%. Comparing base (eeb8689) to head (d604e40).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8667      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207377   207391      +14     
  Branches        73       73              
===========================================
  Hits        166730   166730              
- Misses       40647    40661      +14     

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[TGWDB]

Approving so as not to delay/block. But process wise release PRs should only have the release changes and not other fixes.