Automatically produce a CBMC release twice per month

[tautschnig]

This new workflow will create a pull request proposing a CBMC release every 14th and 28th of each month.

• 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 78.38%. Comparing base (6752c40) to head (b885e7b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8425      +/-   ##
===========================================
+ Coverage    77.86%   78.38%   +0.52%     
===========================================
  Files         1726     1726              
  Lines       189890   188613    -1277     
  Branches     18221    18266      +45     
===========================================
  Hits        147849   147849              
+ Misses       42041    40764    -1277     

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[TGWDB]

I feel like I'd like the descriptions/documentation to be clearer that this creates a new minor release, and only triggers if there have been changes to develop since the last release (of any kind).